Borrowings - Maturities (Details) - USD ($)	Dec. 31, 2016	Dec. 31, 2015
Assets Sold under Agreements to Repurchase [Line Items]
Fair Value of securities pledged, including accrued interest receivable	$ 2,970,880,000	$ 1,909,292,000
Repurchase agreements	$ 2,793,705,000	$ 1,798,813,000
Weighted Average Borrowing Rate - Repurchase Agreements	1.00%	0.64%
Overnight (1 Day or Less) [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Fair Value of securities pledged, including accrued interest receivable	$ 0	$ 0
Repurchase agreements	$ 0	$ 0
Weighted Average Borrowing Rate - Repurchase Agreements	0.00%	0.00%
Between 2 and 30 Days [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Fair Value of securities pledged, including accrued interest receivable	$ 2,284,815,000	$ 1,894,491,000
Repurchase agreements	$ 2,154,766,000	$ 1,789,338,000
Weighted Average Borrowing Rate - Repurchase Agreements	1.01%	0.64%
Between 31 and 90 Days [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Fair Value of securities pledged, including accrued interest receivable	$ 686,065,000	$ 14,801,000
Repurchase agreements	$ 638,939,000	$ 9,475,000
Weighted Average Borrowing Rate - Repurchase Agreements	0.96%	1.19%
Greater Than 90 days [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Fair Value of securities pledged, including accrued interest receivable	$ 0	$ 0
Repurchase agreements	$ 0	$ 0
Weighted Average Borrowing Rate - Repurchase Agreements	0.00%	0.00%