Investment Securities Available for Sale - Schedule of Available for Sale Securities (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 1,426,768	$ 1,417,724
Unrealized Gains	19,577	21,031
Unrealized Losses	(45,975)	(25,674)
Fair Value	1,400,370	1,413,081
Available-for-sale securities
Schedule of Available-for-sale Securities [Line Items]
Investment securities, available for sale held in securitization trusts	48,857	47,922
Agency ARMs
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	89,387	93,331
Unrealized Gains	0	22
Unrealized Losses	(2,812)	(2,281)
Fair Value	86,575	91,072
Agency ARMs \| Freddie Mac
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	31,925	33,623
Unrealized Gains	0	16
Unrealized Losses	(1,100)	(852)
Fair Value	30,825	32,787
Agency ARMs \| Fannie Mae
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	52,933	54,958
Unrealized Gains	0	6
Unrealized Losses	(1,568)	(1,236)
Fair Value	51,365	53,728
Agency ARMs \| Ginnie Mae
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	4,529	4,750
Unrealized Gains	0	0
Unrealized Losses	(144)	(193)
Fair Value	4,385	4,557
Agency Fixed- Rate
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	1,092,120	1,059,532
Unrealized Gains	0	669
Unrealized Losses	(35,635)	(12,916)
Fair Value	1,056,485	1,047,285
Agency Fixed- Rate \| Freddie Mac
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	94,005	20,804
Unrealized Gains	0	0
Unrealized Losses	(2,436)	(736)
Fair Value	91,569	20,068
Agency Fixed- Rate \| Fannie Mae
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	997,759	1,038,363
Unrealized Gains	0	669
Unrealized Losses	(33,190)	(12,174)
Fair Value	964,569	1,026,858
Agency Fixed- Rate \| Ginnie Mae
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	356	365
Unrealized Gains	0	0
Unrealized Losses	(9)	(6)
Fair Value	347	359
Agency IOs
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	25,848	41,367
Unrealized Gains	56	271
Unrealized Losses	(7,519)	(10,459)
Fair Value	18,385	31,179
Agency IOs \| Freddie Mac
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	5,912	8,436
Unrealized Gains	0	19
Unrealized Losses	(2,268)	(2,756)
Fair Value	3,644	5,699
Agency IOs \| Fannie Mae
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	5,002	11,310
Unrealized Gains	4	22
Unrealized Losses	(1,631)	(2,989)
Fair Value	3,375	8,343
Agency IOs \| Ginnie Mae
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	14,934	21,621
Unrealized Gains	52	230
Unrealized Losses	(3,620)	(4,714)
Fair Value	11,366	17,137
Agency RMBS
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	1,207,355	1,194,230
Unrealized Gains	56	962
Unrealized Losses	(45,966)	(25,656)
Fair Value	1,161,445	1,169,536
Non-Agency RMBS
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	98,053	100,291
Unrealized Gains	1,168	1,852
Unrealized Losses	(9)	(18)
Fair Value	99,212	102,125
CMBS
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	121,360	123,203
Unrealized Gains	18,353	18,217
Unrealized Losses	0	0
Fair Value	139,713	141,420
CMBS \| Available-for-sale securities
Schedule of Available-for-sale Securities [Line Items]
Investment securities, available for sale held in securitization trusts	$ 48,900	$ 47,900

The total amount of cash and securities held by third party trustees pursuant to terms of debt instruments or other agreements as of the date of each statement of financial position presented, which can be used by the trustee only to pay the noncurrent portion of specified obligations.

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