Fair Value of Financial Instruments - Fair Values of Corporates by Level and Sector (Details) - USD ($) $ in Millions	Jun. 30, 2015	Dec. 31, 2014
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale debt securities, fair value	$ 4,326.7	$ 4,221.8
Corporate
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale debt securities, fair value	3,098.6	2,831.9
Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale debt securities, fair value	4,326.7	4,221.8
Fair Value, Measurements, Recurring | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale debt securities, fair value	0.0	0.0
Fair Value, Measurements, Recurring | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale debt securities, fair value	2,208.6	2,137.4
Fair Value, Measurements, Recurring | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale debt securities, fair value	2,118.1	2,084.4
Fair Value, Measurements, Recurring | Corporate
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale debt securities, fair value	3,098.6	2,831.9
Fair Value, Measurements, Recurring | Corporate | Consumer
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale debt securities, fair value	584.2	582.5
Fair Value, Measurements, Recurring | Corporate | Energy
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale debt securities, fair value	387.3	344.7
Fair Value, Measurements, Recurring | Corporate | Financial services
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale debt securities, fair value	984.8	896.1
Fair Value, Measurements, Recurring | Corporate | Capital goods
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale debt securities, fair value	283.6	267.3
Fair Value, Measurements, Recurring | Corporate | Transportation
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale debt securities, fair value	156.8	145.4
Fair Value, Measurements, Recurring | Corporate | Utilities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale debt securities, fair value	391.2	337.2
Fair Value, Measurements, Recurring | Corporate | Other
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale debt securities, fair value	310.7	258.7
Fair Value, Measurements, Recurring | Corporate | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale debt securities, fair value	0.0	0.0
Fair Value, Measurements, Recurring | Corporate | Level 1 | Consumer
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale debt securities, fair value	0.0	0.0
Fair Value, Measurements, Recurring | Corporate | Level 1 | Energy
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale debt securities, fair value	0.0	0.0
Fair Value, Measurements, Recurring | Corporate | Level 1 | Financial services
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale debt securities, fair value	0.0	0.0
Fair Value, Measurements, Recurring | Corporate | Level 1 | Capital goods
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale debt securities, fair value	0.0	0.0
Fair Value, Measurements, Recurring | Corporate | Level 1 | Transportation
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale debt securities, fair value	0.0	0.0
Fair Value, Measurements, Recurring | Corporate | Level 1 | Utilities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale debt securities, fair value	0.0	0.0
Fair Value, Measurements, Recurring | Corporate | Level 1 | Other
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale debt securities, fair value	0.0	0.0
Fair Value, Measurements, Recurring | Corporate | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale debt securities, fair value	1,635.3	1,414.7
Fair Value, Measurements, Recurring | Corporate | Level 2 | Consumer
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale debt securities, fair value	228.0	219.9
Fair Value, Measurements, Recurring | Corporate | Level 2 | Energy
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale debt securities, fair value	232.4	181.6
Fair Value, Measurements, Recurring | Corporate | Level 2 | Financial services
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale debt securities, fair value	656.3	574.6
Fair Value, Measurements, Recurring | Corporate | Level 2 | Capital goods
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale debt securities, fair value	161.3	142.0
Fair Value, Measurements, Recurring | Corporate | Level 2 | Transportation
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale debt securities, fair value	36.3	30.5
Fair Value, Measurements, Recurring | Corporate | Level 2 | Utilities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale debt securities, fair value	139.9	117.9
Fair Value, Measurements, Recurring | Corporate | Level 2 | Other
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale debt securities, fair value	181.1	148.2
Fair Value, Measurements, Recurring | Corporate | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale debt securities, fair value	1,463.3	1,417.2
Fair Value, Measurements, Recurring | Corporate | Level 3 | Consumer
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale debt securities, fair value	356.2	362.6
Fair Value, Measurements, Recurring | Corporate | Level 3 | Energy
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale debt securities, fair value	154.9	163.1
Fair Value, Measurements, Recurring | Corporate | Level 3 | Financial services
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale debt securities, fair value	328.5	321.5
Fair Value, Measurements, Recurring | Corporate | Level 3 | Capital goods
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale debt securities, fair value	122.3	125.3
Fair Value, Measurements, Recurring | Corporate | Level 3 | Transportation
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale debt securities, fair value	120.5	114.9
Fair Value, Measurements, Recurring | Corporate | Level 3 | Utilities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale debt securities, fair value	251.3	219.3
Fair Value, Measurements, Recurring | Corporate | Level 3 | Other
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale debt securities, fair value	$ 129.6	$ 110.5