Other Comprehensive Income - Changes in AOCI, Net of Tax, by Component (Detail) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2017
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of period		$ 2,571,617
Amounts reclassified from AOCI due to tax reform	$ (14,700)
Balance at end of period	4,409,194	4,409,194
Unrealized Net Gains (Losses) on Securities Available for Sale
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of period		(18,222)
Other comprehensive income (loss) before reclassifications		(5,744)
Amounts reclassified from AOCI		(411)
Amounts reclassified from AOCI due to tax reform		(5,249)
Net current period other comprehensive income (loss)		(11,404)
Balance at end of period	(29,626)	(29,626)
Unrealized Net Gains (Losses) on Derivative Instruments
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of period		5,254
Other comprehensive income (loss) before reclassifications		(759)
Amounts reclassified from AOCI		(46)
Amounts reclassified from AOCI due to tax reform		958
Net current period other comprehensive income (loss)		153
Balance at end of period	5,407	5,407
Unrecognized Pension and Postretirement Obligations
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of period		(48,401)
Other comprehensive income (loss) before reclassifications		(7)
Amounts reclassified from AOCI		0
Amounts reclassified from AOCI due to tax reform		(10,425)
Net current period other comprehensive income (loss)		(10,432)
Balance at end of period	(58,833)	(58,833)
Total
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of period		(61,369)
Other comprehensive income (loss) before reclassifications		(6,510)
Amounts reclassified from AOCI		(457)
Amounts reclassified from AOCI due to tax reform		(14,716)
Net current period other comprehensive income (loss)		(21,683)
Balance at end of period	$ (83,052)	$ (83,052)