Accumulated Other Comprehensive Loss - Components of AOCI (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Jan. 01, 2018
Accumulated Other Comprehensive Income (Loss)
Beginning balance	$ 2,533,165
Beginning balance after reclassifications	2,533,165
Other comprehensive (loss) income before reclassifications	(19,237)
Amounts reclassified from other comprehensive (loss) income	1,680	$ 1,460
Other comprehensive (loss) income, net	(17,557)
Ending balance	3,245,003
Unrealized Gains and Losses on Available for Sale (AFS) Securities
Accumulated Other Comprehensive Income (Loss)
Beginning balance	(12,004)
Beginning balance after reclassifications	(12,004)		$ (12,484)
Other comprehensive (loss) income before reclassifications	(20,929)
Amounts reclassified from other comprehensive (loss) income	548
Other comprehensive (loss) income, net	(20,381)
Ending balance	(32,865)
Non-credit Impairment Losses on AFS Securities
Accumulated Other Comprehensive Income (Loss)
Beginning balance	(380)
Beginning balance after reclassifications	(380)		(380)
Other comprehensive (loss) income before reclassifications	(268)
Amounts reclassified from other comprehensive (loss) income	(16)
Other comprehensive (loss) income, net	(284)
Ending balance	(664)
Unrealized Gains and (Losses) on Derivatives
Accumulated Other Comprehensive Income (Loss)
Beginning balance	(8,338)
Beginning balance after reclassifications	(8,338)		(8,399)
Other comprehensive (loss) income before reclassifications	1,960
Amounts reclassified from other comprehensive (loss) income	1,036
Other comprehensive (loss) income, net	2,996
Ending balance	(5,403)
Defined Benefit Pension Plan
Accumulated Other Comprehensive Income (Loss)
Beginning balance	(25,283)
Beginning balance after reclassifications	(25,283)		(25,283)
Other comprehensive (loss) income before reclassifications	0
Amounts reclassified from other comprehensive (loss) income	112	$ 59
Other comprehensive (loss) income, net	112
Ending balance	(25,171)
Total Accumulated Other Comprehensive Loss
Accumulated Other Comprehensive Income (Loss)
Beginning balance	(46,005)
Beginning balance after reclassifications	(46,005)		$ (46,546)
Ending balance	(64,103)
Accounting Standards Update 2016-01 | Unrealized Gains and Losses on Available for Sale (AFS) Securities
Accumulated Other Comprehensive Income (Loss)
Reclassification due to adoption of new accounting principle	(480)
Accounting Standards Update 2016-01 | Non-credit Impairment Losses on AFS Securities
Accumulated Other Comprehensive Income (Loss)
Reclassification due to adoption of new accounting principle	0
Accounting Standards Update 2016-01 | Unrealized Gains and (Losses) on Derivatives
Accumulated Other Comprehensive Income (Loss)
Reclassification due to adoption of new accounting principle	0
Accounting Standards Update 2016-01 | Defined Benefit Pension Plan
Accumulated Other Comprehensive Income (Loss)
Reclassification due to adoption of new accounting principle	0
Accounting Standards Update 2016-01 | Total Accumulated Other Comprehensive Loss
Accumulated Other Comprehensive Income (Loss)
Reclassification due to adoption of new accounting principle	(480)
Accounting Standards Update 2017-02 | Unrealized Gains and Losses on Available for Sale (AFS) Securities
Accumulated Other Comprehensive Income (Loss)
Reclassification due to adoption of new accounting principle	0
Accounting Standards Update 2017-02 | Non-credit Impairment Losses on AFS Securities
Accumulated Other Comprehensive Income (Loss)
Reclassification due to adoption of new accounting principle	0
Accounting Standards Update 2017-02 | Unrealized Gains and (Losses) on Derivatives
Accumulated Other Comprehensive Income (Loss)
Reclassification due to adoption of new accounting principle	(61)
Accounting Standards Update 2017-02 | Defined Benefit Pension Plan
Accumulated Other Comprehensive Income (Loss)
Reclassification due to adoption of new accounting principle	0
Accounting Standards Update 2017-02 | Total Accumulated Other Comprehensive Loss
Accumulated Other Comprehensive Income (Loss)
Reclassification due to adoption of new accounting principle	$ (61)