Income Taxes (Details 1) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Reconciliation of income taxes computed at the United States federal statutory tax rates
Tax at U.S. statutory rate									$ 55,955	$ 47,522	$ 34,918
Increase (decrease) in taxes resulting from:
Tax-exempt interest income									(3,595)	(3,467)	(3,377)
BOLI income									(1,524)	(1,622)	(1,264)
Investment tax credits									(1,591)	(1,390)	(1,390)
Amortization of investment in low-income housing tax credits									1,873	1,742	1,734
State income tax expense, net of federal benefit									1,626	1,747	1,378
Revaluation of net deferred tax assets as a result of the Tax Cuts and Jobs Act									14,486	0	0
Other items, net									451	315	(249)
Total income tax expense	$ 30,234	$ 14,199	$ 11,993	$ 11,255	$ 11,461	$ 11,706	$ 11,154	$ 10,526	$ 67,681	$ 44,847	$ 31,750