Borrowings - Narrative (Details) - USD ($)	Mar. 31, 2017	Dec. 31, 2016
Disclosure of Repurchase Agreements [Abstract]
Repurchase agreements	$ 3,050,608,000	$ 2,793,705,000
Fair Value of securities pledged, including accrued interest receivable	3,244,283,000	2,970,880,000
Cash Held on Deposit with Repurchase Agreement Counterparties	$ 4,440,000	$ 10,835,000
Weighted Average Borrowing Rate	1.01%	1.00%
Maximum Amount at Risk	$ 193,000,000
Weighted Average Borrowing Rate - FHLB Advances		0.42%
Collateral Held On FHLB Advances		$ 192,200,000
FHLBC Stock	$ 2,500	$ 3,800,000