Fair Value (Details) (USD $)	Mar. 31, 2014	Dec. 31, 2013
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Carrying value of senior notes	$ 550,000,000	$ 550,000,000
Senior Notes Due 2020 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value of senior notes	310,500,000	316,700,000
Carrying value of senior notes	300,000,000	300,000,000
Amount of difference between fair value and carrying value	10,500,000	16,700,000
Senior Notes Due 2021 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair value of senior notes	233,400,000	234,700,000
Carrying value of senior notes	250,000,000	250,000,000
Amount of difference between fair value and carrying value	$ 16,600,000	$ 15,300,000