Consolidated Statement of Comprehensive Income (Unaudited) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Consolidated Statement Of Comprehensive Income (Loss) [Abstract]
Net income, Before-tax Amount	$ 41,627	$ 28,754	$ 118,155	$ 85,754
Net income, Tax (Expense) Benefit	(15,753)	(10,867)	(44,720)	(32,384)
Net income, Net of Tax Amount	25,874	17,887	73,435	53,370
Unrealized gains on available-for-sale securities:
Unrealized holding gains arising during period, Before-tax Amount	4,927	2,313	37,990	5,426
Unrealized holding gains arising during period, Tax (Expense) Benefit	(1,927)	(870)	(14,488)	(2,143)
Unrealized holding gains arising during period, Net of Tax Amount	3,000	1,443	23,502	3,283
Reclassification adjustment for gains included in net income, Before-tax Amount	(261)	(325)	(922)	(1,877)
Reclassification adjustment for gains included in net income, Tax (Expense) Benefit	101	121	348	724
Reclassification adjustment for gains included in net income, Net of Tax Amount	(160)	(204)	(574)	(1,153)
Net unrealized gains, Before-tax Amount	4,666	1,988	37,068	3,549
Net unrealized gains, Tax (Expense) Benefit	(1,826)	(749)	(14,140)	(1,419)
Net unrealized gains, Net of Tax Amount	2,840	1,239	22,928	2,130
Amortization of losses included in net income on available-for-sale securities transferred to held-to-maturity, Before-tax Amount	663	269	1,601	1,041
Amortization of losses included in net income on available-for-sale securities transferred to held-to-maturity, Tax (Expense) Benefit	(237)	(99)	(596)	(387)
Amortization of losses included in net income on available-for-sale securities transferred to held-to-maturity, Net of Tax Amount	426	170	1,005	654
Amortization of losses included in net income on terminated derivative financial instruments that were previously accounted for as cash flow hedges, Before-tax Amount	466	550	1,426	1,430
Amortization of losses included in net income on terminated derivative financial instruments that were previously accounted for as cash flow hedges, Tax (Expense) Benefit	(181)	(214)	(555)	(556)
Amortization of losses included in net income on terminated derivative financial instruments that were previously accounted for as cash flow hedges, Net of Tax Amount	285	336	871	874
Unrealized losses on derivative financial instruments accounted for as cash flow hedges, Before-tax Amount				(471)
Unrealized losses on derivative financial instruments accounted for as cash flow hedges, Tax (Expense) Benefit				183
Unrealized losses on derivative financial instruments accounted for as cash flow hedges, Net of Tax Amount				(288)
Net cash flow hedge activity, Before-tax Amount		550		959
Net cash flow hedge activity, Tax (Expense) Benefit		(214)		(373)
Net cash flow hedge activity, Net of Tax Amount		336		586
Amortization of prior service cost and actuarial losses included in net periodic pension cost for defined benefit pension plan, Before-tax Amount	167	159	501	478
Amortization of prior service cost and actuarial losses included in net periodic pension cost for defined benefit pension plan, Tax (Expense) Benefit	(65)	(62)	(195)	(186)
Amortization of prior service cost and actuarial losses included in net periodic pension cost for defined benefit pension plan, Net of Tax Amount	102	97	306	292
Total other comprehensive income, Before-tax Amount	5,962	2,966	40,596	6,027
Total other comprehensive income, Tax (Expense) Benefit	(2,309)	(1,124)	(15,486)	(2,365)
Total other comprehensive income, net of tax amount	3,653	1,842	25,110	3,662
Comprehensive income, Before-tax Amount	47,589	31,720	158,751	91,781
Comprehensive income, Tax (Expense) Benefit	(18,062)	(11,991)	(60,206)	(34,749)
Comprehensive income, Net of Tax Amount	$ 29,527	$ 19,729	$ 98,545	$ 57,032