Condensed Statements of Operations and Comprehensive Income (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income
Gain on sale of investments									$ (1,113)	$ (660)	$ (1,463)
Expenses
Compensation and employee benefits									195,056	160,506	151,697
Other operating expenses									54,294	47,045	47,570
Total expenses	$ 81,876	$ 82,829	$ 81,795	$ 73,627	$ 68,366	$ 68,552	$ 65,309	$ 63,344	320,127	265,571	258,714
Income tax benefit	(11,647)	(10,802)	(9,485)	(8,397)	(31,327)	(11,639)	(11,905)	(9,754)	(40,331)	(64,625)	(39,662)
Net Income	$ 49,599	$ 49,336	$ 44,384	$ 38,559	$ 14,956	$ 36,479	$ 33,687	$ 31,255	181,878	116,377	121,131
Comprehensive Income									174,430	122,131	111,760
Parent Company
Income
Dividends from subsidiaries									153,574	119,000	108,350
Gain on sale of investments									0	3	0
Intercompany charges for services									16,523	14,299	12,248
Other income									1,284	225	311
Total income									171,381	133,527	120,909
Expenses
Compensation and employee benefits									20,873	17,864	15,665
Other operating expenses									12,201	10,425	7,701
Total expenses									33,074	28,289	23,366
Income before income tax benefit and equity in undistributed net income of subsidiaries									138,307	105,238	97,543
Income tax benefit									3,773	2,983	4,040
Income before equity in undistributed net income of subsidiaries									142,080	108,221	101,583
Equity in undistributed net income of subsidiaries									39,798	8,156	19,548
Net Income									181,878	116,377	121,131
Comprehensive Income									$ 174,430	$ 122,131	$ 111,760