Financial Derivatives - Narrative (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019
Derivative [Line Items]
Gain expected to be reclassified from AOCI to earnings over next 12 months	$ 5,300,000		$ 5,300,000
Gain (loss) from interest rate swaps designated as cash flow hedges reclassified to earnings	0	$ 0	0	$ 0
Investments held as collateral	0		0		$ 0
Cash posted as collateral					132,129,000
Financial derivatives in net payable position	0		0		0
Notional amount	15,123,582,000		15,123,582,000		14,022,179,000
Exchange Cleared
Derivative [Line Items]
Notional amount	12,400,000,000		12,400,000,000		11,000,000,000.0
Interest Rate Swap
Derivative [Line Items]
Notional amount	15,100,000,000		15,100,000,000		14,000,000,000.0
Cash
Derivative [Line Items]
Cash held as collateral	600,000		600,000		2,700,000
Cash posted as collateral	13,600,000		13,600,000		500,000
Available-for-Sale Securities
Derivative [Line Items]
Cash posted as collateral	$ 212,100,000		$ 212,100,000		$ 131,700,000