Income Taxes (Details 1) - 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
Reconciliation of income taxes computed at the United States federal statutory tax rates
Tax at U.S. statutory rate									$ 47,522	$ 34,918	$ 30,061
Increase (decrease) in taxes resulting from:
Tax-exempt interest income									(3,467)	(3,377)	(3,215)
BOLI income									(1,622)	(1,264)	(1,045)
Investment tax credits									(1,390)	(1,390)	(1,390)
Amortization of investment in low-income housing tax credits									1,742	1,734	1,594
State income tax expense, net of federal benefit									1,747	1,378	921
Decrease to valuation allowance									0	0	(152)
Other items, net									315	(249)	(469)
Total income tax expense	$ 11,461	$ 11,706	$ 11,154	$ 10,526	$ 10,149	$ 7,742	$ 6,842	$ 7,017	$ 44,847	$ 31,750	$ 26,305