Fair Value Measurements - Assets and Liabilities Measured at Fair Value on Recurring Basis (Details) - USD ($)	Sep. 30, 2020		Dec. 31, 2019
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale	$ 5,265,192,000	[1]	$ 5,885,887,000
Total equity securities	31,448,000		32,830,000
Total securities	5,265,192,000		5,885,887,000
Asset-backed Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale	532,253,000	[2]	373,254,000
Mutual Fund
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale	16,157,000
Mutual Funds and Common Stock
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale			17,416,000
Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale	5,233,744		5,853,057
Total equity securities	31,448		32,830
Total securities	5,265,192		5,885,887
Fair Value, Measurements, Recurring | Preferred stock
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total equity securities	15,291		15,414
Fair Value, Measurements, Recurring | Mutual Fund
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total equity securities	16,157
Fair Value, Measurements, Recurring | Mutual Funds and Common Stock
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total equity securities			17,416
Fair Value, Measurements, Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale	61,984		41,839
Total equity securities	15,291		15,414
Total securities	77,275		57,253
Fair Value, Measurements, Recurring | Quoted Prices in Active Markets for Identical Assets (Level 1) | Preferred stock
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total equity securities	15,291		15,414
Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale	5,171,760		5,811,218
Total equity securities	16,157		17,416
Total securities	5,187,917		5,828,634
Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2) | Mutual Fund
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total equity securities	16,157
Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2) | Mutual Funds and Common Stock
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total equity securities			17,416
Fair Value, Measurements, Recurring | Mortgage-related Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale	2,652,996		3,353,735
Fair Value, Measurements, Recurring | Mortgage-related Securities | GSE Certificates
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale	1,218,331		1,552,623
Fair Value, Measurements, Recurring | Mortgage-related Securities | GSE CMOs
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale	1,434,665		1,801,112
Fair Value, Measurements, Recurring | Mortgage-related Securities | Significant Other Observable Inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale	2,652,996		3,353,735
Fair Value, Measurements, Recurring | Mortgage-related Securities | Significant Other Observable Inputs (Level 2) | GSE Certificates
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale	1,218,331		1,552,623
Fair Value, Measurements, Recurring | Mortgage-related Securities | Significant Other Observable Inputs (Level 2) | GSE CMOs
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale	1,434,665		1,801,112
Fair Value, Measurements, Recurring | Other Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale	2,580,748		2,499,322
Fair Value, Measurements, Recurring | Other Securities | U.S. Treasury Obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale	61,984		41,839
Fair Value, Measurements, Recurring | Other Securities | GSE Debentures
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale	993,965		1,094,240
Fair Value, Measurements, Recurring | Other Securities | Asset-backed Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale	532,253		373,254
Fair Value, Measurements, Recurring | Other Securities | Municipal Bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale	26,857		26,892
Fair Value, Measurements, Recurring | Other Securities | Corporate Bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale	876,798		867,182
Fair Value, Measurements, Recurring | Other Securities | Capital Trust Notes
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale	88,891		95,915
Fair Value, Measurements, Recurring | Other Securities | Quoted Prices in Active Markets for Identical Assets (Level 1)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale	61,984		41,839
Fair Value, Measurements, Recurring | Other Securities | Quoted Prices in Active Markets for Identical Assets (Level 1) | U.S. Treasury Obligations
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale	61,984		41,839
Fair Value, Measurements, Recurring | Other Securities | Significant Other Observable Inputs (Level 2)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale	2,518,764		2,457,483
Fair Value, Measurements, Recurring | Other Securities | Significant Other Observable Inputs (Level 2) | GSE Debentures
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale	993,965		1,094,240
Fair Value, Measurements, Recurring | Other Securities | Significant Other Observable Inputs (Level 2) | Asset-backed Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale	532,253		373,254
Fair Value, Measurements, Recurring | Other Securities | Significant Other Observable Inputs (Level 2) | Municipal Bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale	26,857		26,892
Fair Value, Measurements, Recurring | Other Securities | Significant Other Observable Inputs (Level 2) | Corporate Bonds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale	876,798		867,182
Fair Value, Measurements, Recurring | Other Securities | Significant Other Observable Inputs (Level 2) | Capital Trust Notes
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Securities available for sale	$ 88,891		$ 95,915

[1] Excludes accrued interest receivable of $14.0 million included in other assets in the Consolidated Statements of Condition.

[2] The underlying assets of the asset-backed securities are substantially guaranteed by the U.S. Government.