Debt Securities Available-for-Sale - Narrative (Details)	3 Months Ended		9 Months Ended
	Sep. 30, 2018 USD ($)	Sep. 30, 2017 USD ($)	Sep. 30, 2018 USD ($) security	Sep. 30, 2017 USD ($)	Dec. 31, 2017 USD ($)
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities pledged to secure borrowings and deposits	$ 537,100,000		$ 537,100,000
Proceeds from sale of debt securities available-for-sale	2,500,000	$ 0	32,115,000	$ 967,000
Gross realized gains on sales of securities available-for-sale	7,000		183,000	0
Gross realized losses on sales of securities available-for-sale	0		5,000	4,000
Recognized gains (losses) on trading securities portfolio	412,000	337,000	714,000	1,000,000
Debt securities available-for-sale, at estimated fair value	740,518,000		740,518,000		$ 513,782,000
Gross unrealized losses	19,460,000		19,460,000		9,512,000
Impairment charges against earnings	0	$ 0	0	$ 0
Debt securities
Schedule of Available-for-sale Securities [Line Items]
Debt securities available-for-sale, at estimated fair value	183,805,000		183,805,000		68,479,000
Gross unrealized losses	1,044,000		1,044,000		198,000
GSE | Pass-through Certificates
Schedule of Available-for-sale Securities [Line Items]
Debt securities available-for-sale, at estimated fair value	299,471,000		299,471,000		178,295,000
Gross unrealized losses	6,184,000		6,184,000		2,454,000
Corporate bonds | Debt securities
Schedule of Available-for-sale Securities [Line Items]
Debt securities available-for-sale, at estimated fair value	183,527,000		183,527,000		68,130,000
Gross unrealized losses	1,044,000		$ 1,044,000		$ 198,000
Corporate bonds | Debt securities | Non-Investment Grade
Schedule of Available-for-sale Securities [Line Items]
Number of securities (securities) | security			1
Debt securities available-for-sale, at estimated fair value	5,000,000		$ 5,000,000
Gross unrealized losses	$ 17,000		$ 17,000
Continuous Unrealized Loss Position 12 Months Or Longer | Non-GSE | Real Estate Mortgage Investment Conduits (REMICs)
Schedule of Available-for-sale Securities [Line Items]
Number of securities (securities) | security			1
Continuous Unrealized Loss Position Less Than 12 Months | Corporate bonds | Real Estate Mortgage Investment Conduits (REMICs)
Schedule of Available-for-sale Securities [Line Items]
Number of securities (securities) | security			15
Pass Through | Continuous Unrealized Loss Position 12 Months Or Longer | GSE | Real Estate Mortgage Investment Conduits (REMICs)
Schedule of Available-for-sale Securities [Line Items]
Number of securities (securities) | security			32
Pass Through | Continuous Unrealized Loss Position Less Than 12 Months | GSE | Pass-through Certificates
Schedule of Available-for-sale Securities [Line Items]
Number of securities (securities) | security			47
Non Pass Through | Continuous Unrealized Loss Position 12 Months Or Longer | GSE | Real Estate Mortgage Investment Conduits (REMICs)
Schedule of Available-for-sale Securities [Line Items]
Number of securities (securities) | security			18
Non Pass Through | Continuous Unrealized Loss Position Less Than 12 Months | GSE | Real Estate Mortgage Investment Conduits (REMICs)
Schedule of Available-for-sale Securities [Line Items]
Number of securities (securities) | security			40