Financial Derivatives - Narrative (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020
Derivative [Line Items]
Gain expected to be reclassified from AOCI to earnings over next 12 months	$ 5,700,000		$ 5,700,000
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
Financial derivatives in net payable position	0		0		0
Notional amount	15,921,182,000		15,921,182,000		15,447,553,000
Exchange Cleared
Derivative [Line Items]
Notional amount	13,200,000,000		13,200,000,000		12,800,000,000
Interest Rate Swap
Derivative [Line Items]
Notional amount	15,800,000,000		15,800,000,000		15,400,000,000
Cash
Derivative [Line Items]
Cash held as collateral	2,200,000		2,200,000		1,300,000
Cash posted as collateral	6,000,000		6,000,000		11,200,000
Available-for-Sale Securities
Derivative [Line Items]
Cash posted as collateral	$ 169,200,000		$ 169,200,000		$ 201,100,000