Accumulated Other Comprehensive Loss, Net of Tax (Schedule of Accumulated Other Comprehensive Loss) (Details) - USD ($) $ in Thousands	9 Months Ended		12 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	$ 2,886,515	$ 2,701,958	$ 3,207,770	$ 2,886,515	$ 2,701,958
Other comprehensive (loss) income, net of tax			78,328	94,580	(39,121)
Ending Balance			3,234,625	3,207,770	2,886,515
Available For Sale and Transferred Securities [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	(71,374)	(27,947)		(71,374)	(27,947)
Other comprehensive income (loss) before reclassifications	50,179	88,647			(43,427)
Amounts reclassified from accumulated other comprehensive loss	(6)	(22)			0
Other comprehensive (loss) income, net of tax	50,173	88,625			(43,427)
Ending Balance	67,424	17,251			(71,374)
Derivative Instruments
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	(9,313)	(15,016)		(9,313)	(15,016)
Other comprehensive income (loss) before reclassifications	20,667	(4,945)			208
Amounts reclassified from accumulated other comprehensive loss	8,435	5,074			5,495
Other comprehensive (loss) income, net of tax	29,102	129			5,703
Ending Balance	19,918	(9,184)			(9,313)
Defined Benefit Pension and Postretirement Benefit Plans [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	(49,965)	(48,568)		(49,965)	(48,568)
Other comprehensive income (loss) before reclassifications	(3,887)	1,622			(7,122)
Amounts reclassified from accumulated other comprehensive loss	2,940	4,204	2,940	4,204	5,725
Other comprehensive (loss) income, net of tax	(947)	5,826			(1,397)
Ending Balance	(45,086)	(44,139)			(49,965)
AOCI Attributable to Parent [Member]
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Beginning Balance	(130,652)	(91,531)	(36,072)	(130,652)	(91,531)
Other comprehensive income (loss) before reclassifications	66,959	85,324			(50,341)
Amounts reclassified from accumulated other comprehensive loss	11,369	9,256			11,220
Other comprehensive (loss) income, net of tax	78,328	94,580	78,328	94,580	(39,121)
Ending Balance	$ 42,256	$ (36,072)	$ 42,256	$ (36,072)	$ (130,652)