Financial Derivatives - Narrative (Details) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 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	248,293,000		248,293,000		132,129,000
Financial derivatives in net payable position	0		0		0
Notional amount	16,118,981,000		16,118,981,000		14,022,179,000
Cash
Derivative [Line Items]
Cash held as collateral	1,500,000		1,500,000		2,700,000
Cash posted as collateral	17,600,000		17,600,000		500,000
Available-for-Sale Securities
Derivative [Line Items]
Cash posted as collateral	230,700,000		230,700,000		131,700,000
Exchange Cleared
Derivative [Line Items]
Notional amount	13,300,000,000		13,300,000,000		11,000,000,000.0
Interest Rate Swap
Derivative [Line Items]
Notional amount	$ 16,100,000,000		$ 16,100,000,000		$ 14,000,000,000.0