Borrowings - Maturities (Details) - USD ($)	Mar. 31, 2017	Dec. 31, 2016
Assets Sold under Agreements to Repurchase [Line Items]
Fair Value of securities pledged, including accrued interest receivable	$ 3,244,283,000	$ 2,970,880,000
Repurchase agreements	$ 3,050,608,000	$ 2,793,705,000
Weighted Average Borrowing Rate	1.01%	1.00%
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	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	$ 2,499,265,000	$ 2,284,815,000
Repurchase agreements	$ 2,346,488,000	$ 2,154,766,000
Weighted Average Borrowing Rate	1.00%	1.01%
Between 31 and 90 Days [Member]
Assets Sold under Agreements to Repurchase [Line Items]
Fair Value of securities pledged, including accrued interest receivable	$ 745,018,000	$ 686,065,000
Repurchase agreements	$ 704,120,000	$ 638,939,000
Weighted Average Borrowing Rate	1.04%	0.96%
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	0.00%	0.00%