Basis and Summary of Significant Accounting Policies - Recent Accounting Pronouncements (Details) - USD ($) $ in Thousands		3 Months Ended		9 Months Ended
	Jan. 01, 2018	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Revenues		$ 953,071	$ 838,883	$ 2,835,739	$ 2,499,081
Operating Expenses		420,602	374,157	1,199,171	1,033,530
OPERATING PROFIT		57,286	42,689	201,281	190,184
EARNINGS BEFORE INCOME TAXES		53,415	39,221	170,147	178,839
INCOME TAXES		9,695	13,880	35,680	63,863
NET EARNINGS		43,720	25,341	134,467	114,976
Deferred Income Taxes Payable		248,102		248,102		$ 222,592
Customer Deposits and Advance Payments		71,554		71,554		68,060
Total Liabilities		935,392		935,392		964,260
Retained Earnings		1,945,961		1,945,961		1,819,524
Total Shareholders’ Equity		1,763,506		1,763,506		1,728,004
Total Liabilities & Shareholders’ Equity		2,698,898		2,698,898		$ 2,692,264
Comprehensive Income		44,017	26,123	133,752	116,407
ASU 2014-09
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Deferred Revenue, Amortization Period	5 years
Balance Without ASC 606 Adoption
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Operating Expenses		418,928		1,193,819
OPERATING PROFIT		56,924		200,455
EARNINGS BEFORE INCOME TAXES		53,053		169,321
INCOME TAXES		9,606		35,478
NET EARNINGS		43,447		133,843
Deferred Income Taxes Payable		248,522		248,522
Customer Deposits and Advance Payments		70,028		70,028
Total Liabilities		934,286		934,286
Retained Earnings		1,947,067		1,947,067
Total Shareholders’ Equity		1,764,612		1,764,612
Total Liabilities & Shareholders’ Equity		2,698,898		2,698,898
Comprehensive Income		43,744		133,128
Effect of Change Higher/(Lower) | ASU 2014-09
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Operating Expenses		1,674		5,352
OPERATING PROFIT		362		826
EARNINGS BEFORE INCOME TAXES		362		826
INCOME TAXES		89		202
NET EARNINGS		273		624
Deferred Income Taxes Payable		(420)		(420)
Customer Deposits and Advance Payments	$ 2,400	1,526		1,526
Total Liabilities		1,106		1,106
Retained Earnings	$ (2,400)	(1,106)		(1,106)
Total Shareholders’ Equity		(1,106)		(1,106)
Total Liabilities & Shareholders’ Equity		0		0
Comprehensive Income		273		624
Franchise Royalties and Fees
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Revenues		10,153	$ 11,140	35,140	$ 38,165
Franchise Royalties and Fees | Balance Without ASC 606 Adoption
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Revenues		8,118		28,962
Franchise Royalties and Fees | Effect of Change Higher/(Lower) | ASU 2014-09
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Revenues		$ 2,035		$ 6,178