Securities Available for Sale - Gross realized losses (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Schedule of Securities Available-for-sale [Line Items]
Document Period End Date			Sep. 30, 2019
Other-than-temporary impairment charges	$ 1,200,000	$ 200,000	$ 2,500,000	$ 2,300,000
Credit losses recognized into earnings for other-than-temporary impaired securities	0	0
Proceeds from sale of other securities available-for-sale	4,000,000.0	0	10,400,000	500,000
Other asset-backed securities
Schedule of Securities Available-for-sale [Line Items]
Sale of asset-backed securities	1,200,000,000	$ 576,700,000	$ 3,200,000,000	1,500,000,000
Realized gains (losses), net	$ 0			$ 0