Borrowings - Repurchase Agreements Remaining Maturities (Details 2) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Certain characteristics of the Company's repurchase agreements
Repurchase agreements	$ 3,251,686	$ 2,155,644
Repurchase agreements borrowings	3,251,686	2,155,644
Overnight
Certain characteristics of the Company's repurchase agreements
Repurchase agreements	0	0
1 to 29 days
Certain characteristics of the Company's repurchase agreements
Repurchase agreements	1,387,599	1,386,971
30 to 59 days
Certain characteristics of the Company's repurchase agreements
Repurchase agreements	665,656	167,642
60 to 89 days
Certain characteristics of the Company's repurchase agreements
Repurchase agreements	871,819	601,031
90 to 119 days
Certain characteristics of the Company's repurchase agreements
Repurchase agreements	0	0
Greater than or equal to 120 days
Certain characteristics of the Company's repurchase agreements
Repurchase agreements	$ 326,612	$ 0