Fair Value Summary of Quantitative Information about Level 3 Fair Value Measurements (Detail) $ in Millions		6 Months Ended				12 Months Ended
		Jun. 30, 2015 USD ($) Investment		Jun. 30, 2014 USD ($) Investment		Dec. 31, 2014 USD ($) Investment
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair Value	[1],[2]	$ 20,577.4		$ 18,759.2		$ 19,018.0
Fair Value, Inputs, Level 3
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair Value		12.2		71.9		81.3
External Pricing | Fair Value, Inputs, Level 3 | Fixed maturities
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair Value		$ 10.9		$ 27.6		$ 11.6
External Pricing | Fair Value, Inputs, Level 3 | Fixed maturities | Asset-backed Securities | Commercial mortgage-backed securities | Prepayment Rate Zero Percent
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Number of securities | Investment		1		2		1
Fair Value		$ 10.9		$ 27.6		$ 11.6
Valuation Technique		External vendor		External vendor		External vendor
Unobservable Input, prepayment rate		0.00%		0.00%		0.00%
Internal Pricing | Fair Value, Inputs, Level 3 | Equity securities | Nonredeemable preferred stocks
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Number of securities | Investment				1		1
Fair Value				$ 43.8		$ 69.3
Internal Pricing | Fair Value, Inputs, Level 3 | Equity securities | Nonredeemable preferred stocks | Financials
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair Value		$ 0.0		$ 43.8		$ 69.3
Valuation Technique				Multiple of tangible net book value		Multiple of tangible net book value
Unobservable Input, price to book ratio multiple				1.9		2.6
Subtotal Level 3 Securities | Fair Value, Inputs, Level 3
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair Value		10.9		$ 71.4		$ 80.9
Third Party Pricing Exemption Securities | Fair Value, Inputs, Level 3
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair Value		$ 1.3	[3]	$ 0.5	[4]	$ 0.4

[1] Our portfolio reflects the effect of unsettled security transactions and collateral on open derivative positions; at June 30, 2015, $159.7 million was included in "other liabilities," compared to $235.8 million and $31.3 million at June 30, 2014 and December 31, 2014

[2] The total fair value of the portfolio at June 30, 2015 and 2014, and December 31, 2014 included $0.7 billion, $1.1 billion, and $1.9 billion, respectively, of securities held in a consolidated, non-insurance subsidiary of the holding company, net of any unsettled security transactions.

[3] The fair values for these securities were determined with unobservable inputs not reasonably available to us.

[4] {F|ahBzfndlYmZpbGluZ3MtaHJkcmoLEgZYTUxEb2MiXlhCUkxEb2NHZW5JbmZvOjg5NDMwZjc3MGI5YjRlOTFiNzk1Y2NlMGM3MTg5MmU2fFRleHRTZWxlY3Rpb246OTMxRDBGODExQ0YxQ0E3Mjk3RDkwNkVDQUFFMkY5NUMM}