Accumulated Other Comprehensive Loss - Components of AOCI (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Accumulated Other Comprehensive Income (Loss)
Beginning balance			$ 2,377,156
Other comprehensive income before reclassifications	$ 1,044		2,591
Amounts reclassified from other comprehensive income	1,363	$ 2,152	2,823	$ 3,480
Other comprehensive income, net	2,407		5,414
Ending balance	2,423,901		2,423,901
Unrealized Gains and Losses on Available for Sale (AFS) Securities
Accumulated Other Comprehensive Income (Loss)
Beginning balance	(8,774)		(10,094)
Other comprehensive income before reclassifications	1,896		3,203
Amounts reclassified from other comprehensive income	(13)		0
Other comprehensive income, net	1,883		3,203
Ending balance	(6,891)		(6,891)
Non-credit Impairment Losses on AFS Securities
Accumulated Other Comprehensive Income (Loss)
Beginning balance	(616)		(642)
Other comprehensive income before reclassifications	21		134
Amounts reclassified from other comprehensive income	(39)		(126)
Other comprehensive income, net	(18)		8
Ending balance	(634)		(634)
Unrealized Gains and (Losses) on Derivatives
Accumulated Other Comprehensive Income (Loss)
Beginning balance	(10,862)		(12,464)
Other comprehensive income before reclassifications	(873)		(746)
Amounts reclassified from other comprehensive income	1,356		2,831
Other comprehensive income, net	483		2,085
Ending balance	(10,379)		(10,379)
Defined Benefit Pension Plan
Accumulated Other Comprehensive Income (Loss)
Beginning balance	(18,834)		(18,893)
Other comprehensive income before reclassifications	0		0
Amounts reclassified from other comprehensive income	59	$ 43	118	$ 86
Other comprehensive income, net	59		118
Ending balance	(18,775)		(18,775)
Total Accumulated Other Comprehensive Loss
Accumulated Other Comprehensive Income (Loss)
Beginning balance	(39,086)		(42,093)
Ending balance	$ (36,679)		$ (36,679)