Accumulated Other Comprehensive Loss, Net of Tax (Schedule of Reclassifications from AOCI) (Details) - USD ($)	3 Months Ended								9 Months Ended		12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Reclassification out of Accumulated Other Comprehensive Income [Line Items]
Gain on the sale of investment securities, net											$ 29,000	$ 0	$ 0
Interest expense	$ (50,750,000)	$ (53,597,000)	$ (50,470,000)	$ (44,639,000)	$ (43,261,000)	$ (37,991,000)	$ (35,481,000)	$ (31,753,000)			(199,456,000)	(148,486,000)	(117,318,000)
Interest income	282,000,000	294,136,000	292,257,000	286,190,000	280,392,000	268,363,000	260,491,000	245,921,000			1,154,583,000	1,055,167,000	913,605,000
Income tax expense	(25,966,000)	(25,411,000)	(26,451,000)	(26,141,000)	(26,697,000)	(13,700,000)	(20,743,000)	(20,075,000)			(103,969,000)	(81,215,000)	(98,351,000)
Earnings applicable to common shareholders	$ 88,066,000	$ 91,442,000	$ 96,193,000	$ 97,549,000	$ 96,666,000	$ 97,460,000	$ 79,489,000	$ 78,083,000			372,985,000	351,703,000	246,831,000
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											29,000	0	0
Available For Sale and Transferred Securities [Member]
Reclassification out of Accumulated Other Comprehensive Income [Line Items]
Net of tax									$ 22,000	$ 0			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											(7,000)	0	0
Earnings applicable to common shareholders											22,000	0	0
Derivative Instruments
Reclassification out of Accumulated Other Comprehensive Income [Line Items]
Net of tax									(5,074,000)	(5,495,000)			(4,384,000)
Derivative Instruments | Reclassification out of accumualted comprehensive income [Member]
Reclassification out of Accumulated Other Comprehensive Income [Line Items]
Interest expense											(5,509,000)	(7,425,000)	(7,160,000)
Interest income											(1,323,000)	0	0
Income tax expense											1,758,000	1,930,000	2,776,000
Earnings applicable to common shareholders											(5,074,000)	(5,495,000)	(4,384,000)
Accumulated Defined Benefit Plans Adjustment, Net Unamortized Gain (Loss) [Member]
Reclassification out of Accumulated Other Comprehensive Income [Line Items]
Total before tax											(5,706,000)	(7,708,000)	(6,612,000)
Defined benefit pension and postretirement benefit plans [Member]
Reclassification out of Accumulated Other Comprehensive Income [Line Items]
Tax benefit											1,502,000	1,983,000	2,575,000
Net of tax									$ (4,204,000)	$ (5,725,000)	$ (4,204,000)	$ (5,725,000)	$ (4,037,000)