Fair Value Measurements - Schedule of Fair Values of Financial Liabilities (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Level 1 [Member]
Liabilities:
Long-term debt, including current portion	$ 0	$ 0
Preferred Stock	0	0
Total liabilities at fair value	0	0
Level 2 [Member]
Liabilities:
Long-term debt, including current portion	1,246.3	1,196.7
Preferred Stock	50.1	49.2
Total liabilities at fair value	1,296.4	1,245.9
Level 3 [Member]
Liabilities:
Long-term debt, including current portion	0	0
Preferred Stock	0	0
Total liabilities at fair value	0	0
Estimate of Fair Value, Fair Value Disclosure [Member]
Liabilities:
Long-term debt, including current portion	1,246.3	1,196.7
Preferred Stock	50.1	49.2
Total liabilities at fair value	1,296.4	1,245.9
Carrying Value [Member]
Liabilities:
Long-term debt, including current portion	1,227.6	1,167.3
Preferred Stock	48.5	48.4
Total liabilities at fair value	$ 1,276.1	$ 1,215.7