Derivative Instruments, Offsetting Liabilities (Details) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016
Offsetting Derivative Liabilities [Abstract]
Gross amounts of recognized assets or liabilities	$ 773,957	$ 595,309
Gross amounts offset in the consolidated balance sheet	0	0
Net amounts of assets presented in the consolidated balance sheet	773,957	595,309
Gross amounts not offset in the consolidated balance sheet in financial instruments	(766,872)	(573,730)
Gross amounts not offset in the consolidated balance sheet in cash collateral received (pledged)	(7,085)	(21,579)
Net amount	0	0
Repurchase Agreements [Member]
Offsetting Derivative Liabilities [Abstract]
Gross amounts of recognized assets or liabilities	773,317	594,615
Gross amounts offset in the consolidated balance sheet	0	0
Net amounts of assets presented in the consolidated balance sheet	773,317	594,615
Gross amounts not offset in the consolidated balance sheet in financial instruments	(767,450)	(574,181)
Gross amounts not offset in the consolidated balance sheet in cash collateral received (pledged)	(5,867)	(20,434)
Net amount	0	0
Interest Rate Swaps [Member]
Offsetting Derivative Liabilities [Abstract]
Gross amounts of recognized assets or liabilities	413	339
Gross amounts offset in the consolidated balance sheet	0	0
Net amounts of assets presented in the consolidated balance sheet	413	339
Gross amounts not offset in the consolidated balance sheet in financial instruments	0	0
Gross amounts not offset in the consolidated balance sheet in cash collateral received (pledged)	(413)	(339)
Net amount	0	0
TBAs [Member]
Offsetting Derivative Liabilities [Abstract]
Gross amounts of recognized assets or liabilities	83	75
Gross amounts offset in the consolidated balance sheet	0	0
Net amounts of assets presented in the consolidated balance sheet	83	75
Gross amounts not offset in the consolidated balance sheet in financial instruments	(83)	(75)
Gross amounts not offset in the consolidated balance sheet in cash collateral received (pledged)	0	0
Net amount	0	0
Treasury Futures [Member]
Offsetting Derivative Liabilities [Abstract]
Gross amounts of recognized assets or liabilities	144	280
Gross amounts offset in the consolidated balance sheet	0	0
Net amounts of assets presented in the consolidated balance sheet	144	280
Gross amounts not offset in the consolidated balance sheet in financial instruments	661	526
Gross amounts not offset in the consolidated balance sheet in cash collateral received (pledged)	(805)	(806)
Net amount	$ 0	$ 0