Accumulated Other Comprehensive Loss, Net of Tax (Schedule of Reclassifications from AOCI) (Details) - USD ($) $ in Thousands	3 Months Ended								9 Months Ended		12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Reclassification out of Accumulated Other Comprehensive Income [Line Items]
Gain on the sale of investment securities, net											$ 8	$ 29	$ 0
Interest expense	$ (19,136)	$ (19,983)	$ (27,868)	$ (43,669)	$ (50,750)	$ (53,597)	$ (50,470)	$ (44,639)			(110,656)	(199,456)	(148,486)
Interest income	236,065	239,239	252,275	274,470	282,000	294,136	292,257	286,190			1,002,049	1,154,583	1,055,167
Income tax expense	$ (15,118)	$ (18,289)	$ (14,802)	$ (11,144)	$ (25,966)	$ (25,411)	$ (26,451)	$ (26,141)			(59,353)	(103,969)	(81,215)
Earnings applicable to common shareholders											211,474	372,985	351,703
AOCI, Accumulated Gain (Loss), Debt Securities, Available-for-sale, Including Noncontrolling Interest [Member] | Reclassification out of accumualted comprehensive income [Member]
Reclassification out of Accumulated Other Comprehensive Income [Line Items]
Gain on the sale of investment securities, net											8	29	0
Available For Sale and Transferred Securities [Member]
Reclassification out of Accumulated Other Comprehensive Income [Line Items]
Net of tax									$ 6	$ 22			0
Available For Sale and Transferred Securities [Member] | Reclassification out of accumualted comprehensive income [Member]
Reclassification out of Accumulated Other Comprehensive Income [Line Items]
Income tax expense											(2)	(7)	0
Earnings applicable to common shareholders											6	22	0
Derivative Instruments
Reclassification out of Accumulated Other Comprehensive Income [Line Items]
Net of tax									(8,435)	(5,074)			(5,495)
Derivative Instruments | Reclassification out of accumualted comprehensive income [Member]
Reclassification out of Accumulated Other Comprehensive Income [Line Items]
Interest expense											(7,884)	(5,509)	(7,425)
Interest income											(3,536)	(1,323)	0
Income tax expense											2,985	1,758	1,930
Earnings applicable to common shareholders											(8,435)	(5,074)	(5,495)
Accumulated Defined Benefit Plans Adjustment, Net Unamortized Gain (Loss) [Member]
Reclassification out of Accumulated Other Comprehensive Income [Line Items]
Total before tax											(3,976)	(5,706)	(7,708)
Defined benefit pension and postretirement benefit plans [Member]
Reclassification out of Accumulated Other Comprehensive Income [Line Items]
Tax benefit											1,036	1,502	1,983
Net of tax									$ (2,940)	$ (4,204)	$ (2,940)	$ (4,204)	$ (5,725)