Condensed Financial Information (Parent Company Only) - Condensed Statements of Cash Flows (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Cash flows from operating activities:
Net income	$ 106,521	$ 95,636	$ 86,795
Adjustments to reconcile net income to cash provided by operating activities:
Stock-based compensation expense	3,868	4,376	3,959
Tax benefits from stock-based compensation	0	2,146	1,443
Excess tax benefits from stock-based compensation	0	(1,566)	(1,184)
Net cash provided by operating activities	154,683	118,070	115,400
Cash flows from investing activities:
Acquisition of intangible assets	(28,013)	0	0
Acquisition of bank and bank holding company, net of cash and cash equivalents, received	91,779	18,554	(1,636)
Net cash provided by (used in) investing activities	(130,565)	(143,092)	(95,339)
Cash flows from financing activities:
Proceeds from issuance of common stock, net of stock issuance costs	2,431	4,683	3,369
Excess tax benefits from stock-based compensation	0	1,566	1,184
Purchase and retirement of common stock	(1,348)	(26,854)	(20,647)
Dividends paid to common stockholders	(48,583)	(39,353)	(36,290)
Net cash used in financing activities	(47,155)	26,588	(38,274)
Net change in cash and cash equivalents	(23,037)	1,566	(18,213)
Cash and cash equivalents at beginning of period	782,023	780,457	798,670
Cash and cash equivalents at end of period	758,986	782,023	780,457
Parent Company
Cash flows from operating activities:
Net income	106,500	95,600	86,800
Adjustments to reconcile net income to cash provided by operating activities:
Undistributed earnings of subsidiaries	11,000	32,400	(27,400)
Stock-based compensation expense	3,900	4,400	3,900
Tax benefits from stock-based compensation	0	2,100	1,400
Excess tax benefits from stock-based compensation	0	(1,600)	(1,200)
Other, net	14,700	(800)	(11,300)
Net cash provided by operating activities	136,100	132,100	52,200
Cash flows from investing activities:
Acquisition of bank and bank holding company, net of cash and cash equivalents, received	(128,300)	0	(7,200)
Investment in subsidiary	(18,000)	0	0
Net cash provided by (used in) investing activities	(156,300)	2,000	(7,200)
Cash flows from financing activities:
Net (decrease) increase in advances from nonbank subsidiaries	(28,400)	9,100	(2,000)
Repayment of long-term debt	0	0	(1,000)
Proceeds from issuance of common stock, net of stock issuance costs	2,400	4,700	3,400
Excess tax benefits from stock-based compensation	0	1,600	1,200
Purchase and retirement of common stock	(1,300)	(26,900)	(20,600)
Dividends paid to common stockholders	(48,600)	(39,400)	(36,300)
Net cash used in financing activities	(75,900)	(50,900)	(55,300)
Net change in cash and cash equivalents	(96,100)	83,200	(10,300)
Cash and cash equivalents at beginning of period	138,400	55,200	65,500
Cash and cash equivalents at end of period	42,300	138,400	55,200
Nonbank subsidiaries | Parent Company
Cash flows from investing activities:
Capital distributions from nonbank subsidiaries	18,000	2,000	0
Acquisition of intangible assets	(28,000)	$ 0	$ 0
BOTC Acquisition
Cash flows from financing activities:
Stock Issued	$ 386,000