Fair value measurements (Details 3) (USD $)	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010
Significant Unobservable inputs (Level 3) | Pension benefits
Changes in Level 3 assets
Balance at the beginning of the period	$ 141,000	$ 67,420,000
Realized and unrealized gains	92,000	6,650,000
Purchases and settlements, net	(16,000)	(317,000)
Transfer in or out of Level 3		(73,612,000)
Balance at the end of the period	217,000	141,000
Significant Unobservable inputs (Level 3) | Other benefits
Changes in Level 3 assets
Balance at the beginning of the period	5,000	13,703,000
Realized and unrealized gains	3,000	1,445,000
Purchases and settlements, net	(1,000)	(3,854,000)
Transfer in or out of Level 3		(11,289,000)
Balance at the end of the period	7,000	5,000
Fair value measurements on a recurring basis | Quoted prices in active markets for identical assets (Level 1) | Pension benefits
Fair value measurements on a recurring and nonrecurring basis
Total assets	606,000,000	597,000,000
Fair value measurements on a recurring basis | Quoted prices in active markets for identical assets (Level 1) | Pension benefits | Equity securities
Fair value measurements on a recurring and nonrecurring basis
Fair value of plan assets	425,000,000	453,000,000
Fair value measurements on a recurring basis | Quoted prices in active markets for identical assets (Level 1) | Pension benefits | Equity index funds
Fair value measurements on a recurring and nonrecurring basis
Fair value of plan assets	82,000,000	80,000,000
Fair value measurements on a recurring basis | Quoted prices in active markets for identical assets (Level 1) | Pension benefits | Fixed income securities
Fair value measurements on a recurring and nonrecurring basis
Fair value of plan assets	98,000,000	55,000,000
Fair value measurements on a recurring basis | Quoted prices in active markets for identical assets (Level 1) | Pension benefits | Pooled and mutual funds
Fair value measurements on a recurring and nonrecurring basis
Fair value of plan assets	1,000,000	9,000,000
Fair value measurements on a recurring basis | Quoted prices in active markets for identical assets (Level 1) | Other benefits
Fair value measurements on a recurring and nonrecurring basis
Total assets	125,000,000	135,000,000
Fair value measurements on a recurring basis | Quoted prices in active markets for identical assets (Level 1) | Other benefits | Equity securities
Fair value measurements on a recurring and nonrecurring basis
Fair value of plan assets	73,000,000	80,000,000
Fair value measurements on a recurring basis | Quoted prices in active markets for identical assets (Level 1) | Other benefits | Equity index funds
Fair value measurements on a recurring and nonrecurring basis
Fair value of plan assets	15,000,000	14,000,000
Fair value measurements on a recurring basis | Quoted prices in active markets for identical assets (Level 1) | Other benefits | Fixed income securities
Fair value measurements on a recurring and nonrecurring basis
Fair value of plan assets	37,000,000	2,000,000
Fair value measurements on a recurring basis | Quoted prices in active markets for identical assets (Level 1) | Other benefits | Pooled and mutual funds
Fair value measurements on a recurring and nonrecurring basis
Fair value of plan assets		39,000,000
Fair value measurements on a recurring basis | Significant other observable inputs (Level 2) | Pension benefits
Fair value measurements on a recurring and nonrecurring basis
Total assets	271,000,000	252,000,000
Fair value measurements on a recurring basis | Significant other observable inputs (Level 2) | Pension benefits | Fixed income securities
Fair value measurements on a recurring and nonrecurring basis
Fair value of plan assets	185,000,000	183,000,000
Fair value measurements on a recurring basis | Significant other observable inputs (Level 2) | Pension benefits | Pooled and mutual funds
Fair value measurements on a recurring and nonrecurring basis
Fair value of plan assets	86,000,000	69,000,000
Fair value measurements on a recurring basis | Significant other observable inputs (Level 2) | Other benefits
Fair value measurements on a recurring and nonrecurring basis
Total assets	19,000,000	16,000,000
Fair value measurements on a recurring basis | Significant other observable inputs (Level 2) | Other benefits | Fixed income securities
Fair value measurements on a recurring and nonrecurring basis
Fair value of plan assets	6,000,000	6,000,000
Fair value measurements on a recurring basis | Significant other observable inputs (Level 2) | Other benefits | Pooled and mutual funds
Fair value measurements on a recurring and nonrecurring basis
Fair value of plan assets	13,000,000	10,000,000
Fair value measurements on a recurring basis | Estimated fair value | Pension benefits
Fair value measurements on a recurring and nonrecurring basis
Total assets	877,000,000	849,000,000
Receivables and payables, net	(37,000,000)	(17,000,000)
Fair value of plan assets	840,000,000	832,000,000
Fair value measurements on a recurring basis | Estimated fair value | Pension benefits | Equity securities
Fair value measurements on a recurring and nonrecurring basis
Fair value of plan assets	425,000,000	453,000,000
Fair value measurements on a recurring basis | Estimated fair value | Pension benefits | Equity index funds
Fair value measurements on a recurring and nonrecurring basis
Fair value of plan assets	82,000,000	80,000,000
Fair value measurements on a recurring basis | Estimated fair value | Pension benefits | Fixed income securities
Fair value measurements on a recurring and nonrecurring basis
Fair value of plan assets	283,000,000	238,000,000
Fair value measurements on a recurring basis | Estimated fair value | Pension benefits | Pooled and mutual funds
Fair value measurements on a recurring and nonrecurring basis
Fair value of plan assets	87,000,000	78,000,000
Fair value measurements on a recurring basis | Estimated fair value | Other benefits
Fair value measurements on a recurring and nonrecurring basis
Total assets	144,000,000	151,000,000
Receivables and payables, net	(1,000,000)
Fair value of plan assets	143,000,000	151,000,000
Fair value measurements on a recurring basis | Estimated fair value | Other benefits | Equity securities
Fair value measurements on a recurring and nonrecurring basis
Fair value of plan assets	73,000,000	80,000,000
Fair value measurements on a recurring basis | Estimated fair value | Other benefits | Equity index funds
Fair value measurements on a recurring and nonrecurring basis
Fair value of plan assets	15,000,000	14,000,000
Fair value measurements on a recurring basis | Estimated fair value | Other benefits | Fixed income securities
Fair value measurements on a recurring and nonrecurring basis
Fair value of plan assets	43,000,000	8,000,000
Fair value measurements on a recurring basis | Estimated fair value | Other benefits | Pooled and mutual funds
Fair value measurements on a recurring and nonrecurring basis
Fair value of plan assets	$ 13,000,000	$ 49,000,000