Financial Derivatives - Narrative (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Dec. 31, 2017
Derivative [Line Items]
Gain (Loss) on Discontinuation of Interest Rate Cash Flow Hedge Due to Forecasted Transaction Probable of Not Occurring, Net	$ 0
Securities Purchased under Agreements to Resell	0
Financial derivatives, at fair value	22,570	$ 26,599
Derivative, Collateral [Abstract]
Collateral Pledged	25,253	24,926
Additional Collateral, Aggregate Fair Value	0	3,100
Notional Disclosures [Abstract]
Derivative, Notional Amount	9,734,245	8,860,296
Uncollateralized [Member]
Derivative [Line Items]
Credit exposure to interest rate swap counterparties	1,600	500
Cash [Member]
Derivative, Collateral [Abstract]
Collateral Received	0	0
Collateral Pledged	400	100
Securities Pledged as Collateral [Member]
Derivative, Collateral [Abstract]
Collateral Pledged	24,900	24,800
Exchange Cleared [Member]
Notional Disclosures [Abstract]
Derivative, Notional Amount	8,600,000	7,900,000
Interest Rate Swap [Member]
Notional Disclosures [Abstract]
Derivative, Notional Amount	9,700,000	8,800,000
Including accrued interest [Member] | Excluding netting [Member]
Derivative [Line Items]
Credit exposure to interest rate swap counterparties	59,500	28,500
Financial derivatives, at fair value	62,200	58,200
Including accrued interest [Member] | Netting [Member]
Derivative [Line Items]
Credit exposure to interest rate swap counterparties	1,600	500
Financial derivatives, at fair value	$ 4,800	$ 28,000