Accumulated Other Comprehensive Loss - Components of AOCI (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Accumulated Other Comprehensive Income (Loss)
Beginning balance			$ 2,377,156
Other comprehensive income before reclassifications	$ 1,432		4,023
Amounts reclassified from other comprehensive income	1,147	$ 2,093	3,970	$ 5,573
Other comprehensive income, net	2,579		7,993
Ending balance	2,537,984		2,537,984
Unrealized Gains and Losses on Available for Sale (AFS) Securities
Accumulated Other Comprehensive Income (Loss)
Beginning balance	(6,891)		(10,094)
Other comprehensive income before reclassifications	1,457		4,660
Amounts reclassified from other comprehensive income	(4)		(4)
Other comprehensive income, net	1,453		4,656
Ending balance	(5,438)		(5,438)
Non-credit Impairment Losses on AFS Securities
Accumulated Other Comprehensive Income (Loss)
Beginning balance	(634)		(642)
Other comprehensive income before reclassifications	(223)		(89)
Amounts reclassified from other comprehensive income	(40)		(166)
Other comprehensive income, net	(263)		(255)
Ending balance	(897)		(897)
Unrealized Gains and (Losses) on Derivatives
Accumulated Other Comprehensive Income (Loss)
Beginning balance	(10,379)		(12,464)
Other comprehensive income before reclassifications	198		(548)
Amounts reclassified from other comprehensive income	1,132		3,963
Other comprehensive income, net	1,330		3,415
Ending balance	(9,049)		(9,049)
Defined Benefit Pension Plan
Accumulated Other Comprehensive Income (Loss)
Beginning balance	(18,775)		(18,893)
Other comprehensive income before reclassifications	0		0
Amounts reclassified from other comprehensive income	59	$ 42	177	$ 128
Other comprehensive income, net	59		177
Ending balance	(18,716)		(18,716)
Total Accumulated Other Comprehensive Loss
Accumulated Other Comprehensive Income (Loss)
Beginning balance	(36,679)		(42,093)
Ending balance	$ (34,100)		$ (34,100)