Supplemental Information Concerning Insurance Operations (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014		Dec. 31, 2013		Dec. 31, 2012
Supplemental Information for Property, Casualty Insurance Underwriters [Line Items]
Deferred Policy Acquisition Costs	$ 353.2		$ 334.7		$ 303.5
Reserves for Unpaid Claims and Claim Adjustment Expenses	11,597.2		11,952.5		12,239.8
Discount if Any Deducted, in Reserves for Unpaid Claims and Claim Adjustment Expenses	0		0		0
Unearned Premiums	1,834.2		1,765.6		1,705.3
Earned Premiums	4,410.6		4,239.2		3,733.0
Net Investment Income	448.9		415.2		317.5
Claims and Claim Adjustment Expense Incurred Related to Current Year	2,709.7	[1]	2,682.3	[1]	2,642.6	[1]
Claims and Claim Adjustment Expense Incurred Related to Prior Year	(215.2)		(203.0)		(12.3)	[2]
Amortization of Deferred Policy Acquisition Costs	1,042.0		973.5		578.1
Paid Claims and Claim Adjustment Expenses	2,698.9		2,755.3		2,320.0
Premiums Written	$ 4,497.5		$ 4,287.4		$ 3,723.9

[1] 2012 includes amounts from TransRe from the TransRe Acquisition Date through December 31, 2012, including $87.8 million of favorable prior accident year development on loss and LAE reserves acquired.

[2] $87.8 million of favorable prior accident year development on loss and LAE reserves acquired in the merger are included in current year net loss and LAE for the reinsurance segment.