Senior Notes (Narrative) (Details) (USD $)		9 Months Ended
	May 24, 2013	Sep. 30, 2014 Senior Notes [Member] Senior Notes 4.868% [Member]	Sep. 30, 2014 Senior Notes [Member] Senior Notes 5.4% [Member]	Dec. 31, 2013 Senior Notes [Member] Senior Notes 5.4% [Member]
Debt Instrument [Line Items]
Date issued		Jun. 05, 2014	Oct. 12, 2004
Principal amounts	$ 329,897,000	$ 400,000,000	$ 250,000,000
Debt maturity term		30 years
Debt instrument, stated interest rate		4.868%	5.40%	5.40%
Maturity Date		Jun. 01, 2044	Oct. 15, 2014
Capitalized cost		$ 4,300,000