Fair value measurements Fair value disclosure (Details) (USD $)	3 Months Ended
	Jun. 30, 2012	Mar. 31, 2013	Dec. 31, 2012
Fair Value Disclosures [Abstract]
Deferred Compensation Plan, Final Distribution of Plan Liabilities	$ 14,000,000
Deferred Compensation Plan, Full Surrender Value of Rabbi-Trust Owned Life Insurance Policies	16,200,000
Deferred Compensation Plan Assets		0	0
Deferred Compensation Liability, Current		0	0
Long-term Debt, Fair Value		2,020,000,000	2,024,000,000
Long-term Debt		$ 1,915,147,000	$ 1,917,979,000