Financial Derivatives - Narrative (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Dec. 31, 2016
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	26,599		$ 58,152
Derivative, Collateral [Abstract]
Collateral Pledged	24,926		25,643
Additional Collateral, Aggregate Fair Value	3,100		15,800
Notional Disclosures [Abstract]
Derivative, Notional Amount	8,860,296		8,100,257
Uncollateralized [Member]
Derivative [Line Items]
Credit exposure to interest rate swap counterparties	500		200
Cash [Member]
Derivative, Collateral [Abstract]
Collateral Received	0		0
Collateral Pledged	100		1,000
Securities Pledged as Collateral [Member]
Derivative, Collateral [Abstract]
Collateral Pledged	24,800		24,600
Exchange Cleared [Member]
Notional Disclosures [Abstract]
Derivative, Notional Amount	7,900,000		6,900,000
Interest Rate Swap [Member]
Notional Disclosures [Abstract]
Derivative, Notional Amount	8,800,000		8,100,000
Including accrued interest [Member] | Excluding netting [Member]
Derivative [Line Items]
Credit exposure to interest rate swap counterparties	28,500		24,500
Financial derivatives, at fair value	58,200		65,700
Including accrued interest [Member] | Netting [Member]
Derivative [Line Items]
Credit exposure to interest rate swap counterparties	500		200
Financial derivatives, at fair value	$ 28,000		$ 41,400