Basis of Presentation and Significant Accounting Policies Offsetting Assets and Liabilities (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Offsetting [Abstract]
Gross amount of recognized repurchase agreements	$ 1,475,264	$ 451,167
Gross amount of financial assets offset against repurchase agreements in the balance sheet	0	0
Repurchase agreements	1,475,264	451,167
Gross amount of financial assets not offset against repurchase agreements in the balance sheet	(1,475,264)	(451,167)
Gross amount of cash collateral pledged not offset against repurchase agreements in the balance sheet	0	0
Net amount of repurchase agreements after effects of amounts offset and not offset in the balance sheet	$ 0	$ 0