Accumulated Other Comprehensive Loss, Net of Tax (Schedule of Other Comprehensive Income (Loss)) (Details) - USD ($) $ in Thousands	9 Months Ended			12 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Available-for-sale securities:
Net unrealized loss during the period				$ 120,333	$ (58,792)	$ (12,423)
Reclassification for net gain included in net income				(29)	0	0
Total available-for-sale securities				120,304	(58,792)	(12,423)
Derivative instruments:
Net unrealized loss during the period				(6,672)	280	291
Reclassification adjustment for net loss included in net income				6,832	7,425	7,160
Total derivative instruments				160	7,705	7,451
Defined benefit pension and other postretirement benefit plans:
Current year actuarial gain				2,202	(9,600)	155
Reclassification adjustment for amortization of net loss included in net income				5,706	7,708	6,612
Total (gain) loss recognized in OCI				(7,908)	1,892	(6,767)
Available-for-sale securities:
Net unrealized loss during the period				(31,686)	15,365	4,833
Reclassification for net gain included in net income				(7)	0	0
Total available-for-sale securities				31,679	(15,365)	(4,833)
Derivative instruments:
Net unrealized loss during the period				1,727	(72)	(110)
Reclassification adjustment for net loss included in net income				(1,758)	(1,930)	(2,776)
Total derivative instruments				(31)	(2,002)	(2,886)
Defined benefit pension and other postretirement benefit plans:
Current year actuarial loss				(580)	2,478	(57)
Reclassification adjustment for amortization of net loss included in net income				(1,502)	(1,983)	(2,575)
Total defined benefit pension and postretirement benefit plans				(2,082)	495	(2,632)
Available-for-sale securities:
Net unrealized loss during the period				88,647	(43,427)	(7,590)
Reclassification for net gain included in net income				(22)	0	0
Total available-for-sale securities	$ 88,625	$ (43,427)	$ (7,590)	88,625	(43,427)	(7,590)
Derivative instruments:
Net unrealized loss during the period				(4,945)	208	181
Reclassification adjustment for net loss included in net income				5,074	5,495	4,384
Total derivative instruments				129	5,703	4,565
Defined benefit pension and other postretirement benefit plans:
Current year actuarial loss				1,622	(7,122)	98
Reclassification adjustment for amortization of net loss included in net income				4,204	5,725	4,037
Total defined benefit pension and postretirement benefit plans	5,826	(1,397)	4,135	5,826	(1,397)	4,135
Other Comprehensive Income (Loss), Pre-Tax Amount				128,372	(52,979)	1,795
Other Comprehensive Income (Loss), Tax Benefit (Expense)				(33,792)	13,858	(685)
Other comprehensive (loss) income, net of tax	$ 94,580	$ (39,121)	$ 1,110	$ 94,580	$ (39,121)	$ 1,110