Other Comprehensive Income - Schedule of Changes in Accumulated Other Comprehensive Loss after Tax (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	$ 2,377,156	$ 2,207,091	$ 1,863,017
Other comprehensive income (loss) before reclassifications	(1,296)	(3,039)	(6,036)
Amounts reclassified from other comprehensive income (loss)	5,311	6,641	2,836
Other comprehensive income (loss), net	4,015	3,602	(3,200)
Ending balance	2,533,165	2,377,156	2,207,091
Unrealized Gains and Losses on AFS Securities
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(10,094)	(5,336)	(1,890)
Other comprehensive income (loss) before reclassifications	352	(4,293)	(2,000)
Amounts reclassified from other comprehensive income (loss)	11	(465)	(1,446)
Other comprehensive income (loss), net	363	(4,758)	(3,446)
Ending balance	(12,004)	(10,094)	(5,336)
Non-credit Impairment Losses on Securities
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(642)	(520)	145
Other comprehensive income (loss) before reclassifications	498	417	(241)
Amounts reclassified from other comprehensive income (loss)	(167)	(539)	(424)
Other comprehensive income (loss), net	331	(122)	(665)
Ending balance	(380)	(642)	(520)
Unrealized Gains and Losses on Derivatives
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(12,464)	(17,644)	(14,532)
Other comprehensive income (loss) before reclassifications	576	(2,461)	(7,239)
Amounts reclassified from other comprehensive income (loss)	5,028	7,641	4,127
Other comprehensive income (loss), net	5,604	5,180	(3,112)
Ending balance	(8,338)	(12,464)	(17,644)
Defined Benefit Pension Plan
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(18,893)	(22,195)	(26,218)
Other comprehensive income (loss) before reclassifications	(2,722)	3,298	3,444
Amounts reclassified from other comprehensive income (loss)	439	4	579
Other comprehensive income (loss), net	(2,283)	3,302	4,023
Ending balance	(25,283)	(18,893)	(22,195)
Total Accumulated Other Comprehensive Loss
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning balance	(42,093)	(45,695)	(42,495)
Ending balance	$ (46,005)	(42,093)	$ (45,695)
Adjustments for New Accounting Pronouncement | Unrealized Gains and Losses on AFS Securities
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Reclassification due to the adoption of ASU No. 2018-02		(2,273)
Adjustments for New Accounting Pronouncement | Non-credit Impairment Losses on Securities
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Reclassification due to the adoption of ASU No. 2018-02		(69)
Adjustments for New Accounting Pronouncement | Unrealized Gains and Losses on Derivatives
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Reclassification due to the adoption of ASU No. 2018-02		(1,478)
Adjustments for New Accounting Pronouncement | Defined Benefit Pension Plan
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Reclassification due to the adoption of ASU No. 2018-02		(4,107)
Adjustments for New Accounting Pronouncement | Total Accumulated Other Comprehensive Loss
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Reclassification due to the adoption of ASU No. 2018-02		$ (7,927)