Condensed Statements of Operations and Comprehensive Income (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Expenses
Compensation and employee benefits									$ 222,753	$ 195,056	$ 160,506
Other operating expenses									62,775	54,294	47,045
Total expenses	$ 95,252	$ 110,675	$ 86,170	$ 82,830	$ 81,876	$ 82,829	$ 81,795	$ 73,627	374,927	320,127	265,571
Income tax benefit	(12,203)	(12,212)	(12,568)	(11,667)	(11,647)	(10,802)	(9,485)	(8,397)	(48,650)	(40,331)	(64,625)
Net Income	$ 57,410	$ 51,610	$ 52,392	$ 49,132	$ 49,599	$ 49,336	$ 44,384	$ 38,559	210,544	181,878	116,377
Comprehensive Income									260,197	174,430	122,131
Parent Company
Income
Dividends from subsidiaries									142,000	153,574	119,000
Intercompany charges for services									20,661	16,523	14,299
Other income									513	1,284	228
Total income									163,174	171,381	133,527
Expenses
Compensation and employee benefits									25,806	20,873	17,864
Other operating expenses									15,118	12,201	10,425
Total expenses									40,924	33,074	28,289
Income before income tax benefit and equity in undistributed net income of subsidiaries									122,250	138,307	105,238
Income tax benefit									4,488	3,773	2,983
Income before equity in undistributed net income of subsidiaries									126,738	142,080	108,221
Equity in undistributed net income of subsidiaries									83,806	39,798	8,156
Net Income									210,544	181,878	116,377
Comprehensive Income									$ 260,197	$ 174,430	$ 122,131