Debt - Redemption Rates (Details)	12 Months Ended
	Dec. 31, 2015
6% Notes due 2020 | Redemption date 2015
Debt Instrument [Line Items]
Redemption price (as a percent)	0.00%
6% Notes due 2020 | Redemption date 2016
Debt Instrument [Line Items]
Redemption price (as a percent)	0.00%
6% Notes due 2020 | Redemption date 2017
Debt Instrument [Line Items]
Redemption price (as a percent)	103.00%
6% Notes due 2020 | Redemption date 2018
Debt Instrument [Line Items]
Redemption price (as a percent)	101.50%
6% Notes due 2020 | Redemption date 2019
Debt Instrument [Line Items]
Redemption price (as a percent)	100.00%
6% Notes due 2020 | Redemption date 2020
Debt Instrument [Line Items]
Redemption price (as a percent)	100.00%
CAD Notes | Redemption date 2017
Debt Instrument [Line Items]
Redemption price (as a percent)	103.063%
CAD Notes | Redemption date 2018
Debt Instrument [Line Items]
Redemption price (as a percent)	101.531%
CAD Notes | Redemption date 2019
Debt Instrument [Line Items]
Redemption price (as a percent)	100.00%
CAD Notes | Redemption date 2020
Debt Instrument [Line Items]
Redemption price (as a percent)	100.00%
CAD Notes | Redemption date 2021
Debt Instrument [Line Items]
Redemption price (as a percent)	100.00%
GBP Notes | Redemption date 2017
Debt Instrument [Line Items]
Redemption price (as a percent)	104.594%
GBP Notes | Redemption date 2018
Debt Instrument [Line Items]
Redemption price (as a percent)	103.063%
GBP Notes | Redemption date 2019
Debt Instrument [Line Items]
Redemption price (as a percent)	101.531%
GBP Notes | Redemption date 2020
Debt Instrument [Line Items]
Redemption price (as a percent)	100.00%
GBP Notes | Redemption date 2021
Debt Instrument [Line Items]
Redemption price (as a percent)	100.00%
GBP Notes | Redemption date 2022
Debt Instrument [Line Items]
Redemption price (as a percent)	100.00%
6% Notes | Redemption date 2018
Debt Instrument [Line Items]
Redemption price (as a percent)	103.00%
6% Notes | Redemption date 2019
Debt Instrument [Line Items]
Redemption price (as a percent)	102.00%
6% Notes | Redemption date 2020
Debt Instrument [Line Items]
Redemption price (as a percent)	101.00%
6% Notes | Redemption date 2021
Debt Instrument [Line Items]
Redemption price (as a percent)	100.00%
6% Notes | Redemption date 2022
Debt Instrument [Line Items]
Redemption price (as a percent)	100.00%
6% Notes | Redemption date 2023
Debt Instrument [Line Items]
Redemption price (as a percent)	100.00%
The 5 3/4% Notes | Redemption date 2017
Debt Instrument [Line Items]
Redemption price (as a percent)	102.875%
The 5 3/4% Notes | Redemption date 2018
Debt Instrument [Line Items]
Redemption price (as a percent)	101.917%
The 5 3/4% Notes | Redemption date 2019
Debt Instrument [Line Items]
Redemption price (as a percent)	100.958%
The 5 3/4% Notes | Redemption date 2020
Debt Instrument [Line Items]
Redemption price (as a percent)	100.00%
The 5 3/4% Notes | Redemption date 2021
Debt Instrument [Line Items]
Redemption price (as a percent)	100.00%
The 5 3/4% Notes | Redemption date 2022
Debt Instrument [Line Items]
Redemption price (as a percent)	100.00%
The 5 3/4% Notes | Redemption date 2023
Debt Instrument [Line Items]
Redemption price (as a percent)	100.00%
The 5 3/4% Notes | Redemption date 2024
Debt Instrument [Line Items]
Redemption price (as a percent)	100.00%