Consolidated Statements of Cash Flows - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Operating activities:
Net income	$ 33,893	$ 43,099	$ 16,544
Adjustments to reconcile net income to cash provided by operating activities:
Decrease (increase) in accrued interest receivable	577	2,368	(1,607)
Increase (decrease) in accrued interest payable	578	1,413	(204)
(Gain) loss on derivative instruments, net	(3,044)	5,606	43,128
Loss on sale of investments, net	11,530	4,238	978
Fair value adjustments, net	(75)	(103)	(69)
Amortization of investment premiums, net	157,706	150,729	152,308
Other amortization and depreciation, net	1,287	1,502	5,338
Stock-based compensation expense	1,954	2,709	2,965
Increase (decrease) in other assets and liabilities, net	42	(1,047)	(2,430)
Net cash and cash equivalents provided by operating activities	204,448	210,514	216,951
Investing activities:
Purchase of investments	(1,317,959)	(435,046)	(1,122,970)
Principal payments received on investments	307,133	448,567	494,275
Proceeds from sales of investments	1,073,101	99,284	449,921
Principal payments received on mortgage loans held for investment, net	3,386	4,953	15,570
Payment to acquire interest in limited partnership	0	0	(6,000)
Distributions received from limited partnership	0	10,835	0
Net receipts (payments) on derivatives, including terminations	21,986	(60,588)	(39,929)
Other investing activities	(214)	(37)	(237)
Net cash and cash equivalents provided by (used in) investing activities	87,433	67,968	(209,370)
Financing activities:
Borrowings under repurchase agreements and FHLB advances	84,876,542	40,594,639	23,555,007
Repayments of repurchase agreement borrowings and FHLB advances	(85,209,592)	(40,805,107)	(23,458,697)
Principal payments on non-recourse collateralized financing	(938)	(2,039)	(2,395)
Proceeds from issuance of preferred stock	31,350	548	0
Proceeds from issuance of common stock	47,182	137	166
Cash paid for stock issuance costs	(61)	(201)	0
Cash paid for repurchases of common stock	0	(310)	(41,171)
Payments related to tax withholding for stock-based compensation	(521)	(485)	(557)
Dividends paid	(47,532)	(51,900)	(61,016)
Net cash and cash equivalents used in financing activities	(303,570)	(264,718)	(8,663)
Net (decrease) increase in cash, cash equivalents, and restricted cash	(11,689)	13,764	(1,082)
Cash, cash equivalents, and restricted cash at beginning of period	98,889	85,125	86,207
Cash, cash equivalents, and restricted cash at end of period	87,200	98,889	85,125
Supplemental Disclosure of Cash Activity:
Cash paid for interest	$ 35,851	$ 24,033	$ 19,260