Debt (Narrative) (Details) CAD in Millions	1 Months Ended					3 Months Ended	12 Months Ended
	Sep. 30, 2016	Apr. 30, 2016 USD ($)	Mar. 31, 2016 USD ($)	Sep. 30, 2015 USD ($)	Jul. 31, 2015 CAD	Mar. 27, 2016 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	May 31, 2016 USD ($)
Debt Instrument [Line Items]
Loss on debt extinguishment							$ 118,000	$ 1,099,000	$ 3,942,000
Senior Unsecured Debt
Debt Instrument [Line Items]
Unamortized discount and debt issuance costs							21,242,000	21,106,000
Medium-term Notes
Debt Instrument [Line Items]
Unamortized discount and debt issuance costs							12,500,000	11,800,000
Senior Unsecured Notes | Senior Unsecured Debt
Debt Instrument [Line Items]
Unamortized discount and debt issuance costs							$ 8,700,000	9,300,000
6.80% Medium-term notes due 2018
Debt Instrument [Line Items]
Stated interest rate							6.80%
6.80% Medium-term notes due 2018 | Medium-term Notes
Debt Instrument [Line Items]
Repurchased amount of notes							$ 1,200,000	9,300,000	22,600,000
Stated interest rate							6.80%
Loss on debt extinguishment							$ 100,000	$ 1,100,000	$ 3,900,000
3.88% Medium-term notes due in 2016
Debt Instrument [Line Items]
Stated interest rate								3.88%
3.88% Medium-term notes due in 2016 | Medium-term Notes
Debt Instrument [Line Items]
Extinguishment of debt, amount			$ 450,000,000
Extinguishment of debt, stated interest rate			3.88%
1.15% Medium-term notes due 2015 | Medium-term Notes
Debt Instrument [Line Items]
Extinguishment of debt, amount				$ 600,000,000
Extinguishment of debt, stated interest rate	1.15%
Line of Credit | Global Credit Facilities
Debt Instrument [Line Items]
Amount borrowed from credit facility | CAD					CAD 20
Line of credit facility, outstanding borrowings							$ 0	$ 0
Covenant, debt to equity ratio							10.0
Covenant, debt to equity ratio, excluding subsidiary debt							0.70
Line of Credit | Credit Facility Maturing April 2021, 5 Year Term
Debt Instrument [Line Items]
Debt instrument, term		5 years
Line of credit, maximum borrowing capacity		$ 765,000,000
Line of Credit | Credit Facility Maturing April 2019, 5 Year Term
Debt Instrument [Line Items]
Debt instrument, term							5 years
Line of credit, maximum borrowing capacity							$ 675,000,000
Line of Credit | Credit Facility Maturing April 2017
Debt Instrument [Line Items]
Debt instrument, term						5 years
Line of credit, maximum borrowing capacity						$ 675,000,000
Line of Credit | Credit Facility Expiring May 2017
Debt Instrument [Line Items]
Line of credit, maximum borrowing capacity										$ 25,000,000
Line of credit facility, outstanding borrowings							$ 0
Unsecured Commercial Paper
Debt Instrument [Line Items]
Debt instrument, term							365 days
Weighted-average interest rate of commercial paper							0.93%	0.56%