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Debt Securities Available-for-Sale - Narrative (Details)	3 Months Ended
	Mar. 31, 2019 USD ($) security	Mar. 31, 2018 USD ($)	Dec. 31, 2018 USD ($)
Debt Securities, Available-for-sale [Line Items]
Debt securities, available-for-sale, restricted	$ 484,700,000
Proceeds from sale of debt securities available-for-sale	29,163,000	$ 19,508,000
Debt securities, available-for-sale, realized gain	155,000	60,000
Debt securities, available-for-sale, realized loss	0	5,000
Debt securities, trading, realized gain (loss)	1,100,000	106,000
Debt securities available-for-sale	894,272,000		$ 808,031,000
Gross unrealized losses	8,114,000		13,737,000
Other-than-temporary impairment loss, debt securities, available for sale, recognized in earnings	$ 0	$ 0
Real Estate Mortgage Investment Conduits (REMICs) \| GSE \| Pass Through
Debt Securities, Available-for-sale [Line Items]
Number of debt securities, unrealized loss position longer than 12 months \| security	50
Real Estate Mortgage Investment Conduits (REMICs) \| GSE \| Non Pass Through
Debt Securities, Available-for-sale [Line Items]
Number of debt securities, unrealized loss position less than 12 months \| security	1
Number of debt securities, unrealized loss position longer than 12 months \| security	45
Real Estate Mortgage Investment Conduits (REMICs) \| Non-GSE
Debt Securities, Available-for-sale [Line Items]
Number of debt securities, unrealized loss position longer than 12 months \| security	1
Real Estate Mortgage Investment Conduits (REMICs) \| Municipal bonds
Debt Securities, Available-for-sale [Line Items]
Number of debt securities, unrealized loss position longer than 12 months \| security	7
Real Estate Mortgage Investment Conduits (REMICs) \| Corporate bonds
Debt Securities, Available-for-sale [Line Items]
Number of debt securities, unrealized loss position less than 12 months \| security	8
Pass-through Certificates \| GSE
Debt Securities, Available-for-sale [Line Items]
Debt securities available-for-sale	$ 404,537,000		314,788,000
Gross unrealized losses	$ 2,227,000		$ 3,542,000
Pass-through Certificates \| GSE \| Pass Through
Debt Securities, Available-for-sale [Line Items]
Number of debt securities, unrealized loss position less than 12 months \| security	5
Non-Investment Grade \| Debt securities \| Corporate bonds
Debt Securities, Available-for-sale [Line Items]
Number of securities \| security	1
Debt securities available-for-sale	$ 5,000,000
Gross unrealized losses	$ 13,000