Acquisitions - Fair Value of Net Assets Acquired and Liabilities Assumed (Details) - USD ($)	May 31, 2018	Sep. 30, 2018	Dec. 31, 2017
Business Acquisition [Line Items]
Goodwill		$ 2,312,500,000	$ 917,700,000
TWG Holdings Limited
Business Acquisition [Line Items]
Fixed maturity securities available for sale	$ 2,268,800,000
Equity securities	49,400,000
Short-term investments	165,500,000
Other investments	100,900,000
Cash and cash equivalents	380,100,000
Premiums and accounts receivable, net	274,200,000
Reinsurance recoverables	1,917,500,000
Accrued investment income	31,600,000
Property and equipment	15,400,000
Value of business acquired	4,048,000,000.0
Other intangible assets	459,300,000
Other assets	214,100,000
Unearned premiums and contract fees	(7,598,100,000)
Claims and benefits payable	(420,500,000)
Reinsurance balances payable	(186,100,000)
Funds held under reinsurance	(200,800,000)
Accounts payable and other liabilities	(459,200,000)
Non-controlling interest	(1,800,000)
Total identifiable net assets acquired	1,058,300,000
Goodwill	1,408,000,000.0
Total acquisition consideration	2,466,300,000
Tax-deductible goodwill	0
Value of business acquired	4,048,000,000.0
Amortizable intangible assets	$ 447,700,000
Minimum | TWG Holdings Limited
Business Acquisition [Line Items]
Estimated useful lives (in years)	3 years
Maximum | TWG Holdings Limited
Business Acquisition [Line Items]
Estimated useful lives (in years)	15 years