Fair Value of Financial Instruments - Reconciliation of Level 3 Financial Liabilities (Details) (Fair Value, Measurements, Nonrecurring, Level 3, USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2014		Mar. 31, 2013
Fair Value, Measurements, Nonrecurring | Level 3
Level 3 Financial Liabilities:
Balance, beginning of period	$ 74.8		$ 79.5
Net purchases/(sales)	7.2		1.7
Transfers into Level 3	0		0
Transfers out of Level 3	0		0
Realized (gains) losses	11.7	[1]	0.6	[1]
Balance, end of period	$ 93.7		$ 81.8

[1] Realized gains and losses are included in net realized investment gains on the statements of income and comprehensive income.