Financial Derivatives - Narrative (Details) - USD ($)	6 Months Ended
	Jun. 30, 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	20,164,000	$ 26,599,000
Derivative, Collateral [Abstract]
Collateral Pledged	24,940,000	24,926,000
Additional Collateral, Aggregate Fair Value	0	3,100,000
Notional Disclosures [Abstract]
Derivative, Notional Amount	10,176,118,000	8,860,296,000
Uncollateralized [Member]
Derivative [Line Items]
Credit exposure to interest rate swap counterparties	2,000,000	500,000
Cash [Member]
Derivative, Collateral [Abstract]
Collateral Received	0	0
Collateral Pledged	20,000	100,000
Securities Pledged as Collateral [Member]
Derivative, Collateral [Abstract]
Collateral Pledged	24,900,000	24,800,000
Exchange Cleared [Member]
Notional Disclosures [Abstract]
Derivative, Notional Amount	8,800,000,000	7,900,000,000
Interest Rate Swap [Member]
Notional Disclosures [Abstract]
Derivative, Notional Amount	10,100,000,000	8,800,000,000
Including accrued interest [Member] | Excluding netting [Member]
Derivative [Line Items]
Credit exposure to interest rate swap counterparties	71,500,000	28,500,000
Financial derivatives, at fair value	55,300,000	58,200,000
Including accrued interest [Member] | Netting [Member]
Derivative [Line Items]
Credit exposure to interest rate swap counterparties	2,000,000	500,000
Financial derivatives, at fair value	$ 700,000	$ 28,000,000