Investments in Unconsolidated Entities - Narrative (Details) - USD ($)	3 Months Ended			7 Months Ended	9 Months Ended
	Sep. 30, 2019	Mar. 31, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2019	Sep. 30, 2018	Feb. 26, 2019	Dec. 31, 2018
Schedule of Equity Method Investments [Line Items]
Ownership interest (as a percent)	50.00%			50.00%	50.00%			50.00%
Total revenues	$ 155,500,000		$ 198,700,000		$ 718,100,000	$ 363,100,000
Total assets	3,836,300,000			$ 3,836,300,000	3,836,300,000			$ 3,362,600,000
Total liabilities	765,400,000			765,400,000	765,400,000			644,400,000
MediaAlpha
Schedule of Equity Method Investments [Line Items]
Ownership interest (as a percent)							48.30%
Fully diluted ownership interest (as a percent)							42.00%
Assets, fair value		$ 114,700,000
Unrealized investment gain	35,300,000	$ 114,700,000			150,000,000.0
Total assets	0			0	0			88,400,000
Total liabilities	0			0	0			$ 46,900,000
MediaAlpha
Schedule of Equity Method Investments [Line Items]
Total revenues	0		$ 74,500,000		48,800,000	$ 218,000,000.0
MediaAlpha | MediaAlpha
Schedule of Equity Method Investments [Line Items]
Total revenues	110,500,000			233,200,000
Expenses	102,100,000			217,900,000
Gross profit	18,400,000			38,800,000
Pre-tax income	8,400,000			15,300,000
Total assets	103,400,000			103,400,000	103,400,000
Total liabilities	$ 145,800,000			$ 145,800,000	$ 145,800,000