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Investment Securities Available for Sale - Narrative (Details) - USD ($)	3 Months Ended		12 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018
Schedule of Investments [Line Items]
Proceeds from sales of investment securities	$ 56,800,000	$ 10,100,000
Realized net (loss) gain on sales of investment securities available for sale	$ (16,800,000)	(3,400,000)
Contractual maturities (up to)	30 years
Weighted average life of available for sale securities portfolio	8 years 99 days		5 years 270 days
Gross unrealized losses	$ 22,174,000		$ 40,126,000
Other-than-temporary impairment recorded in earnings	0	$ 0
Agency RMBS
Schedule of Investments [Line Items]
Gross unrealized losses	21,759,000		38,284,000
Non-Agency RMBS
Schedule of Investments [Line Items]
Gross unrealized losses	$ 415,000		$ 1,466,000

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