Consolidated Statements of Comprehensive Income (unaudited) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Statement Of Income And Comprehensive Income [Abstract]
Net income	$ 51,348	$ 39,372	$ 143,332	$ 114,218
Change in debt securities available-for-sale:
Unrealized holding gains (losses) for the period	(18,159)	(3,955)	(51,280)	42,452
Reclassification for securities transferred to held-to-maturity			14,007
Reclassification adjustment for securities gains realized in income	(135)	(2,972)	(2,417)	(7,547)
Income tax effect	4,188	2,513	9,368	(12,764)
Unrealized gains (losses) on available-for-sale debt securities	(14,106)	(4,414)	(30,322)	22,141
Change in securities held-to-maturity:
Adjustment for securities transferred to available-for-sale			19,412
Adjustment for securities transferred from available-for-sale			(14,007)
Amortization of unrealized losses on securities transferred from available-for-sale	628	456	1,740	1,358
Income tax effect	(146)	(156)	(1,291)	(465)
Changes from securities held-to-maturity	482	300	5,854	893
Cash flow hedges:
Net unrealized derivative gains (losses) on cash flow hedges	725	217	6,804	(1,590)
Reclassification adjustment for (gains) losses realized in net income	(226)	1,429	553	4,962
Income tax effect	(122)	(625)	(1,805)	(1,281)
Changes from cash flow hedges	377	1,021	5,552	2,091
Defined benefit pension plans:
Amortization of net loss recognized in income	27	27	81	81
Income tax effect	(7)	(11)	(20)	(31)
Changes from defined benefit pension plans	20	16	61	50
Other comprehensive income (loss), net of tax	(13,227)	(3,077)	(18,855)	25,175
Comprehensive income	$ 38,121	$ 36,295	$ 124,477	$ 139,393