Long-Term Debt and Credit Facilities (Details) (USD $) In Millions, unless otherwise specified	1 Months Ended
	Nov. 30, 2013 item	Jun. 30, 2014	Dec. 31, 2013
Senior Notes
Long-Term Debt
Aggregate principal amount of senior notes issued	$ 250
Number of series of notes issued	4
Total long-term debt outstanding		250	250
Note purchase agreement fees	0.9
3.25% Senior Notes, Series 2013A due November 15, 2018
Long-Term Debt
Interest rate (as a percent)		3.25%	3.25%
Total long-term debt outstanding		15	15
3.90% Senior Notes, Series 2013B due November 15, 2020
Long-Term Debt
Interest rate (as a percent)		3.90%	3.90%
Total long-term debt outstanding		50	50
4.50% Senior Notes, Series 2013C due November 15, 2023
Long-Term Debt
Interest rate (as a percent)		4.50%	4.50%
Total long-term debt outstanding		90	90
4.65% Senior Notes, Series 2013D due November 15, 2025
Long-Term Debt
Interest rate (as a percent)		4.65%	4.65%
Total long-term debt outstanding		$ 95	$ 95