Income Taxes (Details 1) - 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
Reconciliation of income taxes computed at the United States federal statutory tax rates
Tax at U.S. statutory rate									$ 34,918	$ 30,061	$ 16,011
Increase (decrease) in taxes resulting from:
Tax-exempt interest income									(3,377)	(3,215)	(2,765)
BOLI income									(1,264)	(1,045)	(1,430)
Investment tax credits									(1,390)	(1,390)	(1,063)
Amortization of investment in low-income housing tax credits									1,734	1,594	998
State income tax expense, net of federal benefit									1,378	921	383
Decrease to valuation allowance									0	(152)	(164)
Other items, net									(249)	(469)	289
Total income tax expense	$ 10,149	$ 7,742	$ 6,842	$ 7,017	$ 7,361	$ 7,108	$ 5,941	$ 5,895	$ 31,750	$ 26,305	$ 12,259