Accumulated Other Comprehensive Loss (Details) - USD ($) $ in Thousands		3 Months Ended
		Mar. 31, 2020	Mar. 31, 2019
Accumulated Other Comprehensive Income (Loss) Before Tax1 [Roll Forward]
Beginning of period		$ 3,145,312	$ 3,607,359
Cumulative effect of change in accounting principle		(7,530)	0
Other comprehensive income (loss)		(61,265)	37,874
Period end		3,491,241	2,963,804
Net Unrealized Loss Relating to Hedging Activities [Member]
Accumulated Other Comprehensive Income (Loss) Before Tax1 [Roll Forward]
Beginning of period		(30,207)	(29,119)
Cumulative effect of change in accounting principle			(175)
Net unrealized gains (losses)		(1,125)	(3,379)
Amortization - hedging activities	[1]	1,757	786
Other comprehensive income (loss)		632	(2,593)
Period end		(29,575)	(31,887)
Pension and Postretirement Benefits [Member]
Accumulated Other Comprehensive Income (Loss) Before Tax1 [Roll Forward]
Beginning of period		(6,807)	(5,276)
Net actuarial loss		501
Amortization - pension and postretirement benefits	[2]	295	161
Other comprehensive income (loss)		796	161
Period end		(6,011)	(5,115)
Total [Member]
Accumulated Other Comprehensive Income (Loss) Before Tax1 [Roll Forward]
Beginning of period		(186,972)	(316,507)
Cumulative effect of change in accounting principle			(175)
Net unrealized gains (losses)		(86,862)	30,686
Noncredit other-than-temporary impairment losses			(80)
Noncredit losses included in basis of securities sold		20,239
Accretion of noncredit loss		2,805	6,291
Net actuarial loss		501
Noncredit other-than-temporary impairment losses reclassified to credit loss	[3]		30
Amortization - hedging activities	[1]	1,757	786
Amortization - pension and postretirement benefits	[2]	295	161
Other comprehensive income (loss)		(61,265)	37,874
Period end		(248,237)	(278,808)
Available-for-sale Securities [Member] | Net Unrealized Loss on Available-for-Sale Securities [Member]
Accumulated Other Comprehensive Income (Loss) Before Tax1 [Roll Forward]
Beginning of period		(73,922)	(152,958)
Net unrealized gains (losses)		(85,737)	34,065
Other comprehensive income (loss)		(85,737)	34,065
Period end		(159,659)	(118,893)
Held-to-maturity Securities [Member] | Noncredit Portion of Other-Than-Temporary Impairment Losses on Held-to-Maturity Securities [Member]
Accumulated Other Comprehensive Income (Loss) Before Tax1 [Roll Forward]
Beginning of period		(76,036)	(129,154)
Noncredit other-than-temporary impairment losses			(80)
Noncredit losses included in basis of securities sold		20,239
Accretion of noncredit loss		2,805	6,291
Noncredit other-than-temporary impairment losses reclassified to credit loss	[3]		30
Other comprehensive income (loss)		23,044	6,241
Period end		$ (52,992)	$ (122,913)

[1] Recorded in CO bond interest expense.

[2] Recorded in other expenses in the statement of operations.

[3] Recorded in other income (loss) in the statement of operations.