Financial Derivatives - Narrative (Details) - USD ($)	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Dec. 31, 2019
Derivative [Line Items]
Gain expected to be reclassified from AOCI to earnings over next 12 months	$ 5,200,000
Gain (loss) from interest rate swaps designated as cash flow hedges reclassified to earnings	0	$ 0
Investments held as collateral	0		$ 0
Cash posted as collateral	225,404,000		132,129,000
Financial derivatives in net payable position	0		0
Notional amount	14,991,653,000		14,022,179,000
Cash
Derivative [Line Items]
Cash held as collateral	2,500,000		2,700,000
Cash posted as collateral	19,300,000		500,000
Available-for-Sale Securities
Derivative [Line Items]
Cash posted as collateral	206,100,000		131,700,000
Exchange Cleared
Derivative [Line Items]
Notional amount	11,900,000,000		11,000,000,000.0
Interest Rate Swap
Derivative [Line Items]
Notional amount	$ 15,000,000,000.0		$ 14,000,000,000.0