Income Taxes - Income Tax Reconciliation (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Reconciliation of income taxes computed at the United States federal statutory tax rates
Tax at U.S. statutory rate									$ 45,294	$ 39,616	$ 55,955
Increase (decrease) in taxes resulting from:
Tax-exempt interest income									(1,205)	(1,433)	(3,595)
BOLI income									(1,283)	(975)	(1,524)
Investment tax credits									(1,863)	(1,863)	(1,591)
Amortization of investment in low-income housing tax credits									1,575	1,592	1,873
State income tax expense, net of federal benefit									5,509	4,502	1,626
Revaluation of net deferred tax assets as a result of the Tax Cuts and Jobs Act									0	0	14,486
Other items, net									64	288	451
Total income tax expense	$ 9,424	$ 11,132	$ 13,945	$ 13,590	$ 13,098	$ 8,532	$ 10,424	$ 9,673	$ 48,091	$ 41,727	$ 67,681