Regulatory Restrictions (Details) $ in Thousands	3 Months Ended
	Sep. 30, 2020 USD ($)	Dec. 31, 2019 USD ($)	Jan. 01, 2019	Dec. 31, 2009 USD ($)
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Assets	$ 38,432,872	$ 32,548,069		$ 15,000,000
Mortgage finance(1)	9,378,104	8,169,849
Mortgage Finance, Average Balance	9,100,000
Common Equity Tier 1 [Abstract]
CET1, actual amount	2,654,677	2,653,999
Total capital (to risk-weighted assets):
Total capital (to risk weighted assets), actual amount	3,470,192	3,398,345
Tier 1 capital (to risk-weighted assets):
Tier 1 capital (to risk-weighted assets), actual amount	2,914,677	2,912,529
Tier 1 capital (to average assets):
Tier 1 capital (to average assets), actual amount	$ 2,914,677	$ 2,912,529
Banking Regulation, Risk-Based Information [Abstract]
CET1, actual ratio	9.05%	8.88%
Total capital (to risk weighted assets), actual ratio	0.1184	0.1137
Tier 1 capital (to risk-weighted assets), actual ratio	0.0994	0.0975
Tier 1 capital (to average assets), actual ratio	0.0758	0.0842
Bank
Common Equity Tier 1 [Abstract]
CET1, actual amount	$ 2,688,350	$ 2,676,513
CET1 to be well capitalized under prompt corrective action provisions, amount	1,903,117	1,941,522
Total capital (to risk-weighted assets):
Total capital (to risk weighted assets), actual amount	3,345,075	3,262,144
Total capital (to risk weighted assets) to be well capitalized under prompt corrective action provisions, amount	2,927,873	2,986,957
Tier 1 capital (to risk-weighted assets):
Tier 1 capital (to risk-weighted assets), actual amount	2,848,350	2,835,043
Tier 1 capital (to risk weighted assets) to be well capitalized under prompt corrective action provisions, amount	2,342,298	2,389,565
Tier 1 capital (to average assets):
Tier 1 capital (to average assets), actual amount	2,848,350	2,835,043
Tier 1 capital (to average assets) to be well capitalized under prompt corrective action provisions, amount	$ 1,921,127	$ 1,728,988
Banking Regulation, Risk-Based Information [Abstract]
CET1, actual ratio	9.18%	8.96%
CET1 to be well capitalized under prompt corrective action provisions, ratio	6.50%	6.50%
Total capital (to risk weighted assets), actual ratio	0.1142	0.1092
Total capital (to risk weighted assets) to be well capitalized under prompt corrective action provisions, ratio	0.1000	0.1000
Tier 1 capital (to risk-weighted assets), actual ratio	0.0973	0.0949
Tier 1 capital (to risk weighted assets) to be well capitalized under prompt corrective action provisions, ratio	0.0800	0.0800
Tier 1 capital (to average assets), actual ratio	0.0741	0.0820
Tier 1 capital (to average assets) to be well capitalized under prompt corrective action provisions, ratio	0.0500	0.0500
Basel III, Phased-In
Common Equity Tier 1 [Abstract]
CET1 for capital adequacy purposes, amount	$ 2,052,372	$ 2,091,591
Total capital (to risk-weighted assets):
Total capital (to risk weighted assets) for capital adequacy purposes, amount	3,078,558	3,137,926
Tier 1 capital (to risk-weighted assets):
Tier 1 capital (to risk-weighted assets) for capital adequacy purposes, amount	2,492,166	2,540,226
Tier 1 capital (to average assets):
Tier 1 capital (to average assets) for capital adequacy purposes, amount	$ 1,537,325	$ 1,383,640
Banking Regulation, Risk-Based Information [Abstract]
CET1 for capital adequacy purposes, ratio	7.00%	7.00%
Total capital (to risk weighted assets) for capital adequacy purposes, ratio	0.1050	0.1050
Tier 1 capital (to risk-weighted assets) for capital adequacy purposes, ratio	0.0850	0.0850
Tier 1 capital (to average assets) for capital adequacy purposes, ratio	0.0400	0.0400
Tier 1 capital (to average assets) to be well capitalized under prompt corrective action provisions, ratio			0.025
Basel III, Phased-In | Bank
Common Equity Tier 1 [Abstract]
CET1 for capital adequacy purposes, amount	$ 2,049,511	$ 2,090,870
Total capital (to risk-weighted assets):
Total capital (to risk weighted assets) for capital adequacy purposes, amount	3,074,266	3,136,305
Tier 1 capital (to risk-weighted assets):
Tier 1 capital (to risk-weighted assets) for capital adequacy purposes, amount	2,488,692	2,538,913
Tier 1 capital (to average assets):
Tier 1 capital (to average assets) for capital adequacy purposes, amount	$ 1,536,902	$ 1,383,190
Banking Regulation, Risk-Based Information [Abstract]
CET1 for capital adequacy purposes, ratio	7.00%	7.00%
Total capital (to risk weighted assets) for capital adequacy purposes, ratio	0.1050	0.1050
Tier 1 capital (to risk-weighted assets) for capital adequacy purposes, ratio	0.0850	0.0850
Tier 1 capital (to average assets) for capital adequacy purposes, ratio	0.0400	0.0400