Accumulated Other Comprehensive Income, Net of Tax (Schedule of Other Comprehensive Income (Loss)) (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	$ 3,272,928	$ 3,090,242	$ 3,234,625	$ 3,207,770
Net current-period other comprehensive (loss) income, net of tax	(2,384)	66,245	(36,366)	77,517
Ending Balance	3,329,705	3,174,779	3,329,705	3,174,779
Available For Sale and Transferred Securities [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	37,071	1,562	67,424	17,251
Other comprehensive (loss) before reclassifications	(1,473)	61,914	(31,826)	46,231
Amounts reclassified from accumulated other comprehensive (loss)	0	0	0	(6)
Net current-period other comprehensive (loss) income, net of tax	(1,473)	61,914	(31,826)	46,225
Ending Balance	35,598	63,476	35,598	63,476
Derivative Instruments [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	15,546	17,048	19,918	(9,184)
Other comprehensive (loss) before reclassifications	(2,475)	2,186	(7,645)	26,965
Amounts reclassified from accumulated other comprehensive (loss)	823	1,415	1,621	2,868
Net current-period other comprehensive (loss) income, net of tax	(1,652)	3,601	(6,024)	29,833
Ending Balance	13,894	20,649	13,894	20,649
Defined Benefit Pension and Other Postretirement Benefit Plans [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	(44,343)	(43,410)	(45,086)	(44,139)
Other comprehensive (loss) before reclassifications	0	0	0	0
Amounts reclassified from accumulated other comprehensive (loss)	741	730	1,484	1,459
Net current-period other comprehensive (loss) income, net of tax	741	730	1,484	1,459
Ending Balance	(43,602)	(42,680)	(43,602)	(42,680)
Accumulated Other Comprehensive (Loss), Net of Tax
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	8,274	(24,800)	42,256	(36,072)
Other comprehensive (loss) before reclassifications	(3,948)	64,100	(39,471)	73,196
Amounts reclassified from accumulated other comprehensive (loss)	1,564	2,145	3,105	4,321
Net current-period other comprehensive (loss) income, net of tax	(2,384)	66,245	(36,366)	77,517
Ending Balance	$ 5,890	$ 41,445	$ 5,890	$ 41,445