Business Combinations - Acquisitions Accounted for Business Combinations (Detail) - USD ($) $ / shares in Units, shares in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Business Combination, Separately Recognized Transactions [Line Items]
Business Acquisitions Purchase Price Allocation Current Assets Cash And Cash Equivalents	$ 11,054,000		$ 2,094,000
Cash Paid	934,928,000	$ 41,471,000	124,716,000
Stock Issued During Period, Value, Acquisitions	100,000,000
Other Payable	5,462,000	11,708,000	10,664,000
Recorded Earn-Out Payable	77,377,000	6,921,000	4,462,000
Net Assets Acquired	1,117,767,000	60,100,000	140,334,000
Maximum Potential Earn- Out Payable	147,668,000	27,451,000	16,285,000
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Assets, Prepaid Expense and Other Assets	52,917,000
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Property, Plant, and Equipment	5,829,000	69,000	684,000
Goodwill, Fair Value Disclosure	717,710,000	42,172,000	93,376,000
Business Acquisitions Purchase Price Allocation Amortizable Intangible Assets	342,008,000	18,738,000	50,625,000
Business Acquisitions Purchase Price Allocation Noncompete Agreements Gross	3,222,000	721,000	228,000
Business Acquisitions Purchase Price Allocation Other Noncurrent Assets	18,588,000		20,000
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Assets	1,151,328,000	62,301,000	152,106,000
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Liabilities, Other	32,996,000	1,512,000	8,542,000
Business Acquisition Purchase Price Allocation Other Liabilities Assumed	565,000
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Liabilities	33,561,000	2,201,000	11,772,000
National Programs
Business Combination, Separately Recognized Transactions [Line Items]
Goodwill, Fair Value Disclosure	18,700,000	7,200,000	57,900,000
Retail
Business Combination, Separately Recognized Transactions [Line Items]
Goodwill, Fair Value Disclosure	676,900,000	33,100,000	13,100,000
Wholesale Brokerage [Member]
Business Combination, Separately Recognized Transactions [Line Items]
Goodwill, Fair Value Disclosure	5,500,000	1,200,000	(1,200)
Services [Member]
Business Combination, Separately Recognized Transactions [Line Items]
Goodwill, Fair Value Disclosure	16,500,000	700,000	22,400,000
Dealer Associates [Member]
Business Combination, Separately Recognized Transactions [Line Items]
Business Acquisitions Purchase Price Allocation Current Assets Cash And Cash Equivalents	0
Net Assets Acquired	33,100,000
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Assets, Prepaid Expense and Other Assets	552,000
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Property, Plant, and Equipment	13,000
Goodwill, Fair Value Disclosure	21,467,000
Business Acquisitions Purchase Price Allocation Amortizable Intangible Assets	10,986,000
Business Acquisitions Purchase Price Allocation Noncompete Agreements Gross	21,000
Business Acquisitions Purchase Price Allocation Other Noncurrent Assets	226,000
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Assets	33,265,000
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Liabilities, Other	165,000
Business Acquisition Purchase Price Allocation Other Liabilities Assumed	0
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Liabilities	165,000
Dealer Associates [Member] | Retail
Business Combination, Separately Recognized Transactions [Line Items]
Cash Paid	28,825,000
Stock Issued During Period, Value, Acquisitions	0
Other Payable	1,175,000
Recorded Earn-Out Payable	3,100,000
Net Assets Acquired	33,100,000
Maximum Potential Earn- Out Payable	12,125,000
Tower Hill [Member]
Business Combination, Separately Recognized Transactions [Line Items]
Business Acquisitions Purchase Price Allocation Current Assets Cash And Cash Equivalents	0
Net Assets Acquired	21,488,000
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Assets, Prepaid Expense and Other Assets	0
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Property, Plant, and Equipment	0
Goodwill, Fair Value Disclosure	0
Business Acquisitions Purchase Price Allocation Amortizable Intangible Assets	21,468,000
Business Acquisitions Purchase Price Allocation Noncompete Agreements Gross	20,000
Business Acquisitions Purchase Price Allocation Other Noncurrent Assets	0
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Assets	21,488,000
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Liabilities, Other	0
Business Acquisition Purchase Price Allocation Other Liabilities Assumed	0
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Liabilities	0
Tower Hill [Member] | National Programs
Business Combination, Separately Recognized Transactions [Line Items]
Cash Paid	20,300,000
Stock Issued During Period, Value, Acquisitions	0
Other Payable	0
Recorded Earn-Out Payable	1,188,000
Net Assets Acquired	21,488,000
Maximum Potential Earn- Out Payable	7,700,000
HSR [Member]
Business Combination, Separately Recognized Transactions [Line Items]
Business Acquisitions Purchase Price Allocation Current Assets Cash And Cash Equivalents	3,114,000
Net Assets Acquired	22,123,000
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Assets, Prepaid Expense and Other Assets	818,000
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Property, Plant, and Equipment	124,000
Goodwill, Fair Value Disclosure	18,737,000
Business Acquisitions Purchase Price Allocation Amortizable Intangible Assets	5,516,000
Business Acquisitions Purchase Price Allocation Noncompete Agreements Gross	65,000
Business Acquisitions Purchase Price Allocation Other Noncurrent Assets	21,000
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Assets	28,395,000
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Liabilities, Other	5,930,000
Business Acquisition Purchase Price Allocation Other Liabilities Assumed	342,000
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Liabilities	6,272,000
HSR [Member] | National Programs
Business Combination, Separately Recognized Transactions [Line Items]
Cash Paid	20,132,000
Stock Issued During Period, Value, Acquisitions	0
Other Payable	0
Recorded Earn-Out Payable	1,991,000
Net Assets Acquired	22,123,000
Maximum Potential Earn- Out Payable	9,000,000
PDA [Member]
Business Combination, Separately Recognized Transactions [Line Items]
Business Acquisitions Purchase Price Allocation Current Assets Cash And Cash Equivalents	(248,000)
Net Assets Acquired	24,843,000
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Assets, Prepaid Expense and Other Assets	1,762,000
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Property, Plant, and Equipment	310,000
Goodwill, Fair Value Disclosure	16,547,000
Business Acquisitions Purchase Price Allocation Amortizable Intangible Assets	7,700,000
Business Acquisitions Purchase Price Allocation Noncompete Agreements Gross	82,000
Business Acquisitions Purchase Price Allocation Other Noncurrent Assets	6,000
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Assets	26,159,000
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Liabilities, Other	1,093,000
Business Acquisition Purchase Price Allocation Other Liabilities Assumed	223,000
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Liabilities	1,316,000
PDA [Member] | Services [Member]
Business Combination, Separately Recognized Transactions [Line Items]
Cash Paid	15,025,000
Stock Issued During Period, Value, Acquisitions	0
Other Payable	0
Recorded Earn-Out Payable	9,818,000
Net Assets Acquired	24,843,000
Maximum Potential Earn- Out Payable	17,975,000
Opus [Member]
Business Combination, Separately Recognized Transactions [Line Items]
Business Acquisitions Purchase Price Allocation Current Assets Cash And Cash Equivalents	0
Net Assets Acquired	22,984,000
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Assets, Prepaid Expense and Other Assets	1,215,000
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Property, Plant, and Equipment	11,000
Goodwill, Fair Value Disclosure	16,414,000
Business Acquisitions Purchase Price Allocation Amortizable Intangible Assets	5,008,000
Business Acquisitions Purchase Price Allocation Noncompete Agreements Gross	21,000
Business Acquisitions Purchase Price Allocation Other Noncurrent Assets	315,000
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Assets	22,984,000
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Liabilities, Other	0
Business Acquisition Purchase Price Allocation Other Liabilities Assumed	0
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Liabilities	0
Opus [Member] | Retail
Business Combination, Separately Recognized Transactions [Line Items]
Cash Paid	20,400,000
Stock Issued During Period, Value, Acquisitions	0
Other Payable	200,000
Recorded Earn-Out Payable	2,384,000
Net Assets Acquired	22,984,000
Maximum Potential Earn- Out Payable	3,600,000
F&I [Member]
Business Combination, Separately Recognized Transactions [Line Items]
Business Acquisitions Purchase Price Allocation Current Assets Cash And Cash Equivalents	0
Net Assets Acquired	54,471,000
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Assets, Prepaid Expense and Other Assets	999,000
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Property, Plant, and Equipment	34,000
Goodwill, Fair Value Disclosure	36,423,000
Business Acquisitions Purchase Price Allocation Amortizable Intangible Assets	16,611,000
Business Acquisitions Purchase Price Allocation Noncompete Agreements Gross	21,000
Business Acquisitions Purchase Price Allocation Other Noncurrent Assets	383,000
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Assets	54,471,000
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Liabilities, Other	0
Business Acquisition Purchase Price Allocation Other Liabilities Assumed	0
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Liabilities	0
F&I [Member] | Retail
Business Combination, Separately Recognized Transactions [Line Items]
Cash Paid	44,940,000
Stock Issued During Period, Value, Acquisitions	0
Other Payable	410,000
Recorded Earn-Out Payable	9,121,000
Net Assets Acquired	54,471,000
Maximum Potential Earn- Out Payable	19,500,000
Kerxton [Member]
Business Combination, Separately Recognized Transactions [Line Items]
Business Acquisitions Purchase Price Allocation Current Assets Cash And Cash Equivalents	0
Net Assets Acquired	16,746,000
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Assets, Prepaid Expense and Other Assets	663,000
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Property, Plant, and Equipment	10,000
Goodwill, Fair Value Disclosure	12,423,000
Business Acquisitions Purchase Price Allocation Amortizable Intangible Assets	4,712,000
Business Acquisitions Purchase Price Allocation Noncompete Agreements Gross	22,000
Business Acquisitions Purchase Price Allocation Other Noncurrent Assets	419,000
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Assets	18,249,000
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Liabilities, Other	1,503,000
Business Acquisition Purchase Price Allocation Other Liabilities Assumed	0
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Liabilities	1,503,000
Kerxton [Member] | Retail
Business Combination, Separately Recognized Transactions [Line Items]
Cash Paid	13,176,000
Stock Issued During Period, Value, Acquisitions	0
Other Payable	1,490,000
Recorded Earn-Out Payable	2,080,000
Net Assets Acquired	16,746,000
Maximum Potential Earn- Out Payable	2,920,000
Rodman [Member]
Business Combination, Separately Recognized Transactions [Line Items]
Business Acquisitions Purchase Price Allocation Current Assets Cash And Cash Equivalents	0
Net Assets Acquired	35,102,000
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Assets, Prepaid Expense and Other Assets	1,062,000
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Property, Plant, and Equipment	45,000
Goodwill, Fair Value Disclosure	26,572,000
Business Acquisitions Purchase Price Allocation Amortizable Intangible Assets	10,129,000
Business Acquisitions Purchase Price Allocation Noncompete Agreements Gross	51,000
Business Acquisitions Purchase Price Allocation Other Noncurrent Assets	542,000
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Assets	38,401,000
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Liabilities, Other	3,299,000
Business Acquisition Purchase Price Allocation Other Liabilities Assumed	0
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Liabilities	3,299,000
Rodman [Member] | Retail
Business Combination, Separately Recognized Transactions [Line Items]
Cash Paid	31,121,000
Stock Issued During Period, Value, Acquisitions	0
Other Payable	261,000
Recorded Earn-Out Payable	3,720,000
Net Assets Acquired	35,102,000
Maximum Potential Earn- Out Payable	9,850,000
Hays [Member]
Business Combination, Separately Recognized Transactions [Line Items]
Business Acquisitions Purchase Price Allocation Current Assets Cash And Cash Equivalents	0
Net Assets Acquired	724,600,000
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Assets, Prepaid Expense and Other Assets	36,254,000
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Property, Plant, and Equipment	4,936,000
Goodwill, Fair Value Disclosure	456,217,000
Business Acquisitions Purchase Price Allocation Amortizable Intangible Assets	218,600,000
Business Acquisitions Purchase Price Allocation Noncompete Agreements Gross	2,600,000
Business Acquisitions Purchase Price Allocation Other Noncurrent Assets	13,977,000
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Assets	732,584,000
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Liabilities, Other	7,984,000
Business Acquisition Purchase Price Allocation Other Liabilities Assumed	0
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Liabilities	7,984,000
Hays [Member] | Retail
Business Combination, Separately Recognized Transactions [Line Items]
Cash Paid	605,000,000
Stock Issued During Period, Value, Acquisitions	100,000,000
Other Payable	0
Recorded Earn-Out Payable	19,600,000
Net Assets Acquired	724,600,000
Maximum Potential Earn- Out Payable	25,000,000
Other Acquisitions [Member]
Business Combination, Separately Recognized Transactions [Line Items]
Goodwill, Fair Value Disclosure	717,700,000
Other Acquisitions [Member] | Various [Member]
Business Combination, Separately Recognized Transactions [Line Items]
Cash Paid		41,471,000	26,140,000
Other Payable		11,708,000	464,000
Recorded Earn-Out Payable		6,921,000	400,000
Net Assets Acquired		60,100,000	27,004,000
Maximum Potential Earn- Out Payable		27,451,000	7,785,000
Various [Member]
Business Combination, Separately Recognized Transactions [Line Items]
Business Acquisitions Purchase Price Allocation Current Assets Cash And Cash Equivalents	0		0
Net Assets Acquired	37,556,000		27,004,000
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Assets, Prepaid Expense and Other Assets	323,000
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Property, Plant, and Equipment	100,000		77,000
Goodwill, Fair Value Disclosure	22,712,000		19,570,000
Business Acquisitions Purchase Price Allocation Amortizable Intangible Assets	15,085,000		11,075,000
Business Acquisitions Purchase Price Allocation Noncompete Agreements Gross	297,000		117,000
Business Acquisitions Purchase Price Allocation Other Noncurrent Assets	754,000		20,000
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Assets	39,271,000		32,414,000
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Liabilities, Other	1,715,000		5,410,000
Business Acquisition Purchase Price Allocation Other Liabilities Assumed	0
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Liabilities	1,715,000		5,410,000
Various [Member] | Various [Member]
Business Combination, Separately Recognized Transactions [Line Items]
Cash Paid	30,293,000
Stock Issued During Period, Value, Acquisitions	0
Other Payable	1,367,000
Recorded Earn-Out Payable	5,896,000
Net Assets Acquired	37,556,000
Maximum Potential Earn- Out Payable	12,998,000
ADG [Member]
Business Combination, Separately Recognized Transactions [Line Items]
Business Acquisitions Purchase Price Allocation Current Assets Cash And Cash Equivalents	0
Net Assets Acquired	47,575,000
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Assets, Prepaid Expense and Other Assets	1,500,000
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Property, Plant, and Equipment	67,000
Goodwill, Fair Value Disclosure	35,769,000
Business Acquisitions Purchase Price Allocation Amortizable Intangible Assets	9,751,000
Business Acquisitions Purchase Price Allocation Noncompete Agreements Gross	21,000
Business Acquisitions Purchase Price Allocation Other Noncurrent Assets	467,000
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Assets	47,575,000
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Liabilities, Other	0
Business Acquisition Purchase Price Allocation Other Liabilities Assumed	0
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Liabilities	0
ADG [Member] | Retail
Business Combination, Separately Recognized Transactions [Line Items]
Cash Paid	29,471,000
Stock Issued During Period, Value, Acquisitions	0
Other Payable	559,000
Recorded Earn-Out Payable	17,545,000
Net Assets Acquired	47,575,000
Maximum Potential Earn- Out Payable	20,000,000
Servco [Member]
Business Combination, Separately Recognized Transactions [Line Items]
Business Acquisitions Purchase Price Allocation Current Assets Cash And Cash Equivalents	8,188,000
Net Assets Acquired	77,179,000
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Assets, Prepaid Expense and Other Assets	7,769,000
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Property, Plant, and Equipment	179,000
Goodwill, Fair Value Disclosure	54,429,000
Business Acquisitions Purchase Price Allocation Amortizable Intangible Assets	16,442,000
Business Acquisitions Purchase Price Allocation Noncompete Agreements Gross	1,000
Business Acquisitions Purchase Price Allocation Other Noncurrent Assets	1,478,000
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Assets	88,486,000
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Liabilities, Other	11,307,000
Business Acquisition Purchase Price Allocation Other Liabilities Assumed	0
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Liabilities	11,307,000
Servco [Member] | Retail
Business Combination, Separately Recognized Transactions [Line Items]
Cash Paid	76,245,000
Stock Issued During Period, Value, Acquisitions	0
Other Payable	0
Recorded Earn-Out Payable	934,000
Net Assets Acquired	77,179,000
Maximum Potential Earn- Out Payable	$ 7,000,000
Two Thousand Seventeen Acquisition [Member] [Domain]
Business Combination, Separately Recognized Transactions [Line Items]
Business Acquisition, Pro Forma Revenue		$ 1,891,701,000	$ 1,784,776,000
Business Acquisition, Pro Forma Income (Loss) from Continuing Operations before Changes in Accounting and Extraordinary Items, Net of Tax, Per Share, Diluted		$ 1.41	$ 0.92
Weighted Average Basic Shares Outstanding, Pro Forma		272,580	272,278
Pro Forma Weighted Average Shares Outstanding, Diluted		277,586	275,608
Business Acquisition, Pro Forma Income (Loss) from Continuing Operations, Net of Tax		$ 453,397,000	$ 429,490,000
Business Acquisition, Pro Forma Net Income (Loss)		$ 401,908,000	$ 261,133,000
Business Acquisition, Pro Forma Income (Loss) from Continuing Operations, Net of Tax, Per Share, Basic		$ 1.44	$ 0.93