Financial Derivatives - Narrative (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Derivative [Line Items]
Gain (Loss) on Discontinuation of Interest Rate Cash Flow Hedge Due to Forecasted Transaction Probable of Not Occurring, Net	$ 0	$ 0	$ 0
Securities Purchased under Agreements to Resell	0	0
Financial derivatives, at fair value	58,152	77,199
Derivative, Collateral [Abstract]
Collateral Pledged	25,643	37,986
Additional Collateral, Aggregate Fair Value	15,800	45,200
Notional Disclosures [Abstract]
Derivative, Notional Amount	8,100,257	8,429,964
Uncollateralized [Member]
Derivative [Line Items]
Credit exposure to interest rate swap counterparties	200	0
Cash [Member]
Derivative, Collateral [Abstract]
Collateral Received	0	0
Collateral Pledged	1,000	38,000
Securities Pledged as Collateral [Member]
Derivative, Collateral [Abstract]
Collateral Pledged	24,600	0
Exchange Cleared [Member]
Notional Disclosures [Abstract]
Derivative, Notional Amount	6,900,000	6,200,000
Interest Rate Swap [Member]
Notional Disclosures [Abstract]
Derivative, Notional Amount	8,100,000	8,400,000
ERROR in label resolution. | Excluding netting [Member]
Derivative [Line Items]
Credit exposure to interest rate swap counterparties	24,500	6,400
Financial derivatives, at fair value	65,700	90,100
ERROR in label resolution. | Netting [Member]
Derivative [Line Items]
Credit exposure to interest rate swap counterparties	200	0
Financial derivatives, at fair value	$ 41,400	$ 83,200