Accumulated Other Comprehensive Loss, Net of Tax (Schedule of Other Comprehensive Income (Loss)) (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	$ 3,207,770	$ 2,886,515
Net current-period OCI	11,272	28,811
Ending Balance	3,090,242	2,966,255
Available For Sale and Transferred Securities [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	17,251	(71,374)
OCI before reclassifications	(15,683)	27,559
Amounts reclassified from AOCL	(6)	0
Net current-period OCI	(15,689)	27,559
Ending Balance	1,562	(43,815)
Derivative Instruments [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	(9,184)	(9,313)
OCI before reclassifications	24,779	(839)
Amounts reclassified from AOCL	1,453	1,035
Net current-period OCI	26,232	196
Ending Balance	17,048	(9,117)
Defined Benefit Pension and Other Postretirement Benefit Plans [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	(44,139)	(49,965)
OCI before reclassifications	0	0
Amounts reclassified from AOCL	729	1,056
Net current-period OCI	729	1,056
Ending Balance	(43,410)	(48,909)
AOCI Attributable to Parent [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	(36,072)	(130,652)
OCI before reclassifications	9,096	26,720
Amounts reclassified from AOCL	2,176	2,091
Net current-period OCI	11,272	28,811
Ending Balance	$ (24,800)	$ (101,841)