Fair Value Measurements - Narrative (Details) - loan	3 Months Ended	12 Months Ended
	Mar. 31, 2017	Dec. 31, 2016
Valuation methodologies used for assets measured at fair value
Number of held-for-sale non-performing multi-family mortgage loans	1	1
Weighted average discount rate (as a percent)	9.93%	9.94%
Weighted average constant prepayment rate (as a percent)	10.24%	10.63%
Weighted average life	6 years 22 days	6 years
Residential Mortgage Loans
Valuation methodologies used for assets measured at fair value
Composition of impaired loans (as a percent)	90.00%	90.00%
Impaired loans for which fair value adjustment was recognized (as a percent)	91.00%	90.00%
Period past due after which loans are individually evaluated for impairment	180 days
Period past due when loan servicer performs property inspections	45 days
Multi-family and commercial real estate mortgage loans
Valuation methodologies used for assets measured at fair value
Composition of impaired loans (as a percent)	8.00%	8.00%
Impaired loans for which fair value adjustment was recognized (as a percent)	8.00%	9.00%
Home equity lines of credit
Valuation methodologies used for assets measured at fair value
Composition of impaired loans (as a percent)	2.00%	2.00%
Impaired loans for which fair value adjustment was recognized (as a percent)	1.00%	1.00%
Period past due after which loans are individually evaluated for impairment	90 days
Recurring | Residential mortgage-backed securities
Valuation methodologies used for assets measured at fair value
Available-for-sale portfolio comprised of debt securities (as a percent)	89.00%	90.00%
Available-for-sale mortgage-backed securities portfolio comprised of GSE securities (as a percent)	100.00%	99.00%
Recurring | Obligations of GSEs
Valuation methodologies used for assets measured at fair value
Available-for-sale portfolio comprised of debt securities (as a percent)	11.00%	10.00%