Fair Value of Financial Instruments - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	1 Months Ended
	May 31, 2009	Sep. 30, 2014	Dec. 31, 2013
Fair Value Measurements Disclosure [Line Items]
Total available-for-sale investments		$ 2,247.5	$ 2,176.3
Other invested assets		5.3	5.0
Number of Credit Agreements	2
Notes receivable from affiliate		70.0	70.0
Significant Unobservable Inputs (Level 3) [Member]
Fair Value Measurements Disclosure [Line Items]
Total available-for-sale investments		9.3	8.9
Federal Home Loan Bank Stock		4.8
Other invested assets		0.5
Corporate Securities [Member] | Significant Unobservable Inputs (Level 3) [Member]
Fair Value Measurements Disclosure [Line Items]
Number of fixed maturity corporate securities included in Level 3 (securities)		2
Number of Available for Sale Securities, Fair Value Estimated by Present Value of Future Cash Flows		1
Total available-for-sale investments		9.3	8.9
International funds [Member]
Fair Value Measurements Disclosure [Line Items]
Number of fixed maturity corporate securities included in Level 3 (securities)		2
Total available-for-sale investments		76.2	74.2
Affiliated Entity [Member]
Fair Value Measurements Disclosure [Line Items]
Notes receivable from affiliate		$ 70.0