Acquisitions - Additional Information (Details) - USD ($)		3 Months Ended		6 Months Ended
	May 31, 2018	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018	Dec. 31, 2017
Business Acquisition [Line Items]
Goodwill		$ 2,338,000,000.0		$ 2,338,000,000.0		$ 2,321,800,000
Claims and benefits payable		2,722,200,000	$ 3,503,100,000	2,722,200,000	$ 3,503,100,000	$ 2,813,700,000	$ 3,782,200,000
TWG Holdings Limited
Business Acquisition [Line Items]
Total acquisition consideration	$ 2,466,300,000
Aggregate cash consideration	894,900,000
Repayment of pre-existing TWG debt	595,900,000
Share issuance consideration	$ 975,500,000
Common stock shares issued to TWG equityholders	10,400,000
Goodwill				25,700,000
Goodwill	$ 1,438,100,000
Tax-deductible goodwill	0
Amortization of value of business acquired (VOBA)		294,000,000.0	34,200,000	592,400,000	34,200,000
Claims and benefits payable	$ 419,900,000
Amortization of intangible assets		$ 4,600,000	$ 1,400,000	$ 9,100,000	$ 1,400,000