Long-Term Debt Long-Term Debt - Summary of Maturity Dates, Semi-Annual Interest Payment Dates, and Optional Redemption Periods of Outstanding Senior Note Obligations (Details)	6 Months Ended
	Jun. 30, 2013
2019 Notes [Member]
Debt Instrument [Line Items]
Maturity date	Oct. 01, 2019
Interest payment dates	April 1, Oct. 1
Call premium redemption period	Oct. 01, 2014
Make-whole redemption period	2014-10-01T00:00:00
2020 Notes [Member]
Debt Instrument [Line Items]
Maturity date	Oct. 01, 2020
Interest payment dates	April 1, Oct. 1
Call premium redemption period	Oct. 01, 2015
Make-whole redemption period	2015-10-01T00:00:00
Equity offering redemption period	Oct. 01, 2013
2021 Notes [Member]
Debt Instrument [Line Items]
Maturity date	Apr. 01, 2021
Interest payment dates	April 1, Oct. 1
Call premium redemption period	Apr. 01, 2016
Make-whole redemption period	2016-04-01T00:00:00
Equity offering redemption period	Apr. 01, 2014
2022 Notes [Member]
Debt Instrument [Line Items]
Maturity date	Sep. 15, 2022
Interest payment dates	March 15, Sept. 15
Call premium redemption period	Mar. 15, 2017
Make-whole redemption period	2017-03-15T00:00:00
Equity offering redemption period	Mar. 15, 2015
2023 Notes [Member]
Debt Instrument [Line Items]
Maturity date	Apr. 15, 2023
Interest payment dates	April 15, Oct. 15
Make-whole redemption period	2023-01-15T00:00:00