Long-Term Debt (Bank Credit Facility) (Narrative) (Details) (USD $)					12 Months Ended				12 Months Ended		12 Months Ended					12 Months Ended
	Jun. 30, 2013	Dec. 31, 2012	Jun. 30, 2012	Dec. 31, 2011	Jun. 30, 2013 Bank Credit Facility [Member]	Jun. 30, 2013 Bank Credit Facility [Member] Long-term Debt [Member]	Jun. 30, 2013 Bank Credit Facility [Member] Other Assets [Member]	Jun. 30, 2013 Amended Bank Credit Facility [Member]	Jun. 30, 2013 Revolving Credit Facility [Member] Bank Credit Facility [Member]	Jun. 30, 2013 Revolving Credit Facility [Member] Previous Credit Facility [Member]	Jun. 30, 2013 Term Loan [Member] Bank Credit Facility [Member]	Jun. 30, 2013 Same-Day Swing Line Loan [Member]	Jun. 30, 2013 Same-Day Swing Line Loan [Member] Bank Credit Facility [Member]	Jun. 30, 2013 Stand-By Letters Of Credit [Member]	Jun. 30, 2013 Stand-By Letters Of Credit [Member] Bank Credit Facility [Member]	Jun. 30, 2013 Principal Payment Through September 30, 2015 [Member] Term Loan [Member] Bank Credit Facility [Member]	Jun. 30, 2013 Principal Payment From October 1, 2015 Through September 30, 2016 [Member] Term Loan [Member] Bank Credit Facility [Member]
Debt Instrument [Line Items]
Credit facility maximum borrowing capacity					$ 900,000,000				$ 750,000,000	$ 600,000,000	$ 150,000,000		$ 50,000,000		$ 25,000,000
Loan maturity date					Oct. 21, 2015
Credit facility, available increase in maximum borrowing capacity					200,000,000			300,000,000			150,000,000
Credit facility, increase in maximum borrowing capacity									150,000,000
Credit facility, amount outstanding	180,000,000		125,000,000						180,000,000
Term loan period											5 years
Term loan principal payment																1,900,000	3,800,000
Outstanding borrowings interest rate					1.69%
Fair value of amount outstanding on the Credit Facility												0		400,000
Debt issuance cost capitalized					7,900,000
Unamortized debt issuance expense	$ 3,522,000	$ 11,400,000	$ 4,654,000	$ 24,300,000		$ 3,000,000	$ 1,400,000