Senior Notes (Narrative) (Details) - Senior Notes [Member] - USD ($)	9 Months Ended
	Sep. 30, 2015	Dec. 31, 2014
4.868% Senior notes due 6/1/2044 [Member]
Debt Instrument [Line Items]
Date Issued	Jun. 05, 2014
Principal amounts	$ 400,000,000
Debt maturity term	30 years
Debt instrument, stated interest rate	4.868%	4.868%
Date due	Jun. 01, 2044
5.4% Senior notes due 10/15/2014 [Member]
Debt Instrument [Line Items]
Date Issued	Oct. 12, 2004
Principal amounts	$ 250,000,000
Debt instrument, stated interest rate	5.40%
Date due	Oct. 15, 2014