Capital (Accumulated Other Comprehensive Income (Loss)) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Jun. 30, 2020	Dec. 31, 2019	Jun. 30, 2019	Dec. 31, 2018
Accumulated Other Comprehensive Income (Loss) [Line Items]
AOCI balance	$ 3,371,978	$ 4,785,650	$ 3,371,978	$ 4,785,650	$ 3,818,842	$ 4,472,836	$ 5,055,905	$ 5,376,294
Amortization on hedging activities	0	(10)	(149)	(25)
Other Comprehensive (Income) Loss, Defined Benefit Plan, before Tax, after Reclassification Adjustment, Attributable to Parent	171	106	1,208	317
Net Unrealized Gains(Losses)
Accumulated Other Comprehensive Income (Loss) [Line Items]
AOCI balance	144,433	49,510	144,433	49,510	118,049	45,155	55,105	9,887
Net unrealized gains (losses)	26,384	(5,595)	47,574	39,623
Net Unrealized Gains on Hedging Activities[Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
AOCI balance	0	151	0	151	0	149	161	176
Amortization on hedging activities		(10)	(149)	(25)
Pension and Post Retirement Plans
Accumulated Other Comprehensive Income (Loss) [Line Items]
AOCI balance	(3,974)	(1,733)	(3,974)	(1,733)	(4,145)	(5,182)	(1,839)	(2,050)
Other Comprehensive (Income) Loss, Defined Benefit Plan, before Tax, after Reclassification Adjustment, Attributable to Parent	171	106	1,208	317
AOCI Attributable to Parent [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
AOCI balance	140,459	104,008	140,459	104,008	113,904	91,826	114,116	73,146
Net unrealized gains (losses)	26,384	(10,475)	47,574	30,240
Total Noncredit OTTI to credit OTTI		271		330
Amortization on hedging activities		(10)	(149)	(25)
Other Comprehensive (Income) Loss, Defined Benefit Plan, before Tax, after Reclassification Adjustment, Attributable to Parent	171	106	1,208	317
Available-for-sale Securities | Noncredit OTTI Gains(Losses)
Accumulated Other Comprehensive Income (Loss) [Line Items]
AOCI balance	0	56,080	0	56,080	$ 0	$ 51,704	$ 60,689	$ 65,133
Net unrealized gains (losses)		(4,880)		(9,383)
Noncredit OTTI to credit OTTI		$ 271		$ 330
Cumulative Effect, Period of Adoption, Adjustment [Member] | Accounting Standards Update 2016-13 [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
AOCI balance	113		113
Cumulative Effect, Period of Adoption, Adjustment [Member] | Accounting Standards Update 2016-13 [Member] | Net Unrealized Gains(Losses)
Accumulated Other Comprehensive Income (Loss) [Line Items]
AOCI balance	51,704		51,704
Cumulative Effect, Period of Adoption, Adjustment [Member] | Accounting Standards Update 2016-13 [Member] | AOCI Attributable to Parent [Member]
Accumulated Other Comprehensive Income (Loss) [Line Items]
AOCI balance	0		0
Cumulative Effect, Period of Adoption, Adjustment [Member] | Accounting Standards Update 2016-13 [Member] | Available-for-sale Securities | Noncredit OTTI Gains(Losses)
Accumulated Other Comprehensive Income (Loss) [Line Items]
AOCI balance	$ (51,704)		$ (51,704)