Condensed Statements of Operations and Comprehensive Income (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Expenses
Compensation and employee benefits									$ 151,697	$ 134,409	$ 118,571
Other operating expenses									47,160	44,765	41,648
Total expenses	$ 66,717	$ 65,180	$ 64,461	$ 62,356	$ 62,203	$ 59,112	$ 59,945	$ 55,497	258,714	236,757	212,679
Income tax benefit	(9,662)	(10,156)	(10,492)	(9,352)	(8,341)	(9,305)	(7,488)	(8,865)	(39,662)	(33,999)	(35,909)
Net Income	$ 31,041	$ 30,957	$ 30,451	$ 28,682	$ 29,508	$ 29,614	$ 29,335	$ 27,670	121,131	116,127	112,755
Comprehensive Income									111,760	100,372	120,854
Parent Company
Income
Dividends from subsidiaries									108,350	109,000	78,500
Intercompany charges for services									12,248	10,562	9,283
Other income									311	196	199
Total income									120,909	119,758	87,982
Expenses
Compensation and employee benefits									15,665	13,205	10,773
Other operating expenses									7,701	7,313	6,824
Total expenses									23,366	20,518	17,597
Income before income tax benefit and equity in undistributed net income of subsidiaries									97,543	99,240	70,385
Income tax benefit									4,040	3,105	2,919
Income before equity in undistributed net income of subsidiaries									101,583	102,345	73,304
Equity in undistributed net income of subsidiaries									19,548	13,782	39,451
Net Income									121,131	116,127	112,755
Comprehensive Income									$ 111,760	$ 100,372	$ 120,854