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Investment Securities Available for Sale - Narrative (Details) - USD ($)	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Schedule of Investments [Line Items]
Proceeds from sales of investment securities	$ 16,800,000	$ 15,400,000	$ 26,900,000	$ 52,700,000
Realized net losses on sales of investment securities available for sale	8,800,000	600,000	$ 12,300,000	2,300,000
Contractual maturities (up to)			30 years
Weighted average life of available for sale securities portfolio			5 years 339 days		7 years 47 days
Gross unrealized losses	(44,647,000)		$ (44,647,000)		$ (15,215,000)
Other-than-temporary impairment recorded in earnings	0	$ 0	0	$ 0
Agency RMBS
Schedule of Investments [Line Items]
Gross unrealized losses	$ (44,637,000)		$ (44,637,000)		$ (15,197,000)

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Amount of realized gain (loss) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Amount of accumulated unrealized loss on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in unrealized loss position.

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Amount of other-than-temporary impairment (OTTI) on investment in debt security, recognized in earnings.

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Amount of cash inflow from sale of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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