Debt (Narrative) (Details)	1 Months Ended			12 Months Ended
	Nov. 30, 2017 USD ($)	May 31, 2017 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Jun. 25, 2017 USD ($)
Debt Instrument [Line Items]
Loss on debt extinguishment				$ 0	$ 118,000	$ 1,099,000
Senior Unsecured Debt
Debt Instrument [Line Items]
Unamortized discount and debt issuance costs				$ 19,821,000	21,242,000
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
Stated interest rate				6.80%
Loss on debt extinguishment					$ 100,000	$ 1,100,000
2.70% Medium-term notes due in 2017
Debt Instrument [Line Items]
Stated interest rate					2.70%
2.70% Medium-term notes due in 2017 | Medium-term Notes
Debt Instrument [Line Items]
Extinguishment of debt, amount			$ 400,000,000
Extinguishment of debt, stated interest rate			2.70%
1.55% Medium-term notes due in 2017
Debt Instrument [Line Items]
Stated interest rate					1.55%
1.55% Medium-term notes due in 2017 | Medium-term Notes
Debt Instrument [Line Items]
Extinguishment of debt, amount	$ 400,000,000
Extinguishment of debt, stated interest rate	1.55%
Line of Credit | Global Credit Facilities
Debt Instrument [Line Items]
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 2018, 364 Day Term [Member]
Debt Instrument [Line Items]
Debt instrument, term		364 days
Line of credit, maximum borrowing capacity		$ 100,000,000
Line of Credit | Credit Facility Expiring May 2018 [Member]
Debt Instrument [Line Items]
Line of credit, maximum borrowing capacity							$ 25,000,000
Unsecured Commercial Paper
Debt Instrument [Line Items]
Debt instrument, term				365 days
Weighted-average interest rate of commercial paper				1.48%	0.93%