Condensed Statements of Operations and Comprehensive Income (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Income
Loss on sale of investments									$ 19	$ (188)	$ (299)
Expenses
Compensation and employee benefits									134,409	118,571	104,221
Other operating expenses									44,765	41,648	38,856
Total non-interest expense	$ 62,203	$ 59,112	$ 59,945	$ 55,497	$ 55,698	$ 54,238	$ 52,673	$ 50,070	236,757	212,679	195,317
Income tax benefit	(8,341)	(9,305)	(7,488)	(8,865)	(8,846)	(9,800)	(8,350)	(8,913)	(33,999)	(35,909)	(30,017)
Net Income	$ 29,508	$ 29,614	$ 29,335	$ 27,670	$ 28,054	$ 29,294	$ 28,677	$ 26,730	116,127	112,755	95,644
Total Comprehensive Income									100,372	120,854	57,327
Parent Company
Income
Dividends from subsidiaries									109,000	78,500	65,445
Loss on sale of investments									0	0	(3,248)
Other income									196	199	966
Intercompany charges for services									10,562	9,283	7,387
Total income									119,758	87,982	70,550
Expenses
Compensation and employee benefits									13,205	10,773	9,175
Other operating expenses									7,313	6,824	6,536
Total non-interest expense									20,518	17,597	15,711
Income before income tax benefit and equity in undistributed net income of subsidiaries									99,240	70,385	54,839
Income tax benefit									3,105	2,919	3,676
Income before equity in undistributed net income of subsidiaries									102,345	73,304	58,515
Equity in undistributed net income of subsidiaries									13,782	39,451	37,129
Net Income									116,127	112,755	95,644
Total Comprehensive Income									$ 100,372	$ 120,854	$ 57,327