Consolidated Statements of Comprehensive Income - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Statement of Comprehensive Income [Abstract]
Net income	$ 69,594	$ 68,988	$ 132,536	$ 140,584
Other comprehensive income (loss), net of tax:
Unrealized holding gains (losses) on securities carried at fair value arising during the period	36,809	(11,622)	74,253	(39,229)
Reclassification adjustment for (gains) losses included in net income	(4,160)	(3)	(4,247)	(3)
Tax effect	(6,856)	2,441	(14,701)	8,239
Net unrealized gains (losses) on securities carried at fair value, net of tax	25,793	(9,184)	55,305	(30,993)
Unrealized gains (losses) on interest rate swaps designated as cash flow hedges	(21,229)	4,102	(28,781)	12,065
Reclassification adjustment for (gains) losses included in net income	(1,256)	(589)	(2,544)	(347)
Tax effect	4,722	(738)	6,578	(2,461)
Net unrealized (losses) gains on interest rate swaps designated as cash flow hedges, net of tax	(17,763)	2,775	(24,747)	9,257
Adjustment for pension and other postretirement benefits	96	142	191	284
Tax effect	(20)	(30)	(40)	(60)
Net adjustment for pension and other postretirement benefits	76	112	151	224
Other comprehensive income (loss), net of tax	8,106	(6,297)	30,709	(21,512)
Total comprehensive income, net of tax	$ 77,700	$ 62,691	$ 163,245	$ 119,072