Income Taxes - Reconciliation of Effective Income Tax (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
Income Tax Disclosure [Abstract]
Federal statutory tax rate (as a percent)									21.00%	21.00%	35.00%
Income tax expense at Federal statutory rate									$ 17,020	$ 13,710	$ 21,051
Tax-exempt instruments									(1,745)	(1,879)	(3,212)
Non-deductible acquisition costs									0	336	210
Federal tax credits and other benefits									(1,961)	(515)	(1,510)
Effects of BOLI									(368)	(330)	(531)
Revaluation of net deferred tax assets									0	0	2,568
Other, net									542	(84)	(220)
Income tax expense	$ 3,445	$ 3,621	$ 3,202	$ 3,220	$ 4,690	$ 3,146	$ 2,003	$ 1,399	$ 13,488	$ 11,238	$ 18,356