Financial Derivatives - Narrative (Details) - USD ($)	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018
Derivative [Line Items]
Gain expected to be reclassified from AOCI to earnings over next 12 months	$ 1,300,000
Gain (loss) from interest rate swaps designated as cash flow hedges reclassified to earnings	0	$ 0
Investments held as collateral	900,000		$ 1,100,000
Cash posted as collateral	90,420,000		47,018,000
Financial derivatives in net payable position	0		0
Notional amount	11,808,479,000		9,955,745,000
Uncollateralized [Member]
Derivative [Line Items]
Credit exposure to interest rate swap counterparties	200,000		1,400,000
Cash [Member]
Derivative [Line Items]
Cash held as collateral	3,000,000		700,000
Cash posted as collateral	0		0
Available-for-sale Securities [Member]
Derivative [Line Items]
Cash posted as collateral	90,400,000		47,000,000
Exchange Cleared [Member]
Derivative [Line Items]
Notional amount	9,900,000,000		8,500,000,000
Interest Rate Swap [Member]
Derivative [Line Items]
Notional amount	$ 11,800,000,000		$ 9,900,000,000