Comprehensive Income - Components of Accumulated Other Comprehensive Income (Detail) - USD ($) $ in Thousands	3 Months Ended				6 Months Ended
	Jun. 30, 2020	Mar. 31, 2020	Jun. 30, 2019	Mar. 31, 2019	Jun. 30, 2020	Jun. 30, 2019
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	$ 3,343,702	$ 3,363,833	$ 3,286,891	$ 3,251,624	$ 3,363,833	$ 3,251,624
Net current-period other comprehensive income, net of tax	27,675	18,698	18,903	17,699	46,373	36,602
Ending Balance	4,197,855	3,343,702	3,333,858	3,286,891	4,197,855	3,333,858
Pension Plan [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance		(42,825)			(42,825)
Other comprehensive income before reclassification					0
Amounts reclassified from accumulated other comprehensive income					2,225
Net current-period other comprehensive income, net of tax					2,225
Ending Balance	(40,600)				(40,600)
Unrealized Gains/Losses on AFS Securities [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance		7,956			7,956
Other comprehensive income before reclassification					46,679
Amounts reclassified from accumulated other comprehensive income					(1,259)
Net current-period other comprehensive income, net of tax					45,420
Ending Balance	53,376				53,376
Accumulated Gain (Loss) Net Cash Flow Hedge Parent [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance		0			0
Other comprehensive income before reclassification					(1,272)
Amounts reclassified from accumulated other comprehensive income					0
Net current-period other comprehensive income, net of tax					(1,272)
Ending Balance	(1,272)				(1,272)
Accumulated Other Comprehensive Income (Loss) [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	(16,171)	(34,869)	(39,270)	(57,019)	(34,869)	(57,019)
Other comprehensive income before reclassification					45,407
Amounts reclassified from accumulated other comprehensive income					966
Net current-period other comprehensive income, net of tax	27,675	18,698	18,903	17,699	46,373
Ending Balance	$ 11,504	$ (16,171)	$ (20,367)	$ (39,270)	$ 11,504	$ (20,367)