Fair value measurements Fair value measurements (Details) (USD $)	12 Months Ended
Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Schedule of available-for-sale securities [Line Items]
Investments used to satisfy obligations under nonqualified benefit plans	$ 48,900,000	$ 38,400,000
Net unrealized gain (loss) of investments used to satify obligations under nonqualified benefit plans	5,200,000	(1,100,000)	5,800,000
Available-for-sale securities:
Available-for-sale securities [Abstract]
Available-for-sale securities, amortized cost basis	47,067,000	53,109,000
Available-for-sale securities, gross unrealized gains	11,835,000	6,600,000
Available-for-sale securities, gross unrealized losses	(3,000)	(5,000)
Available-for-sale securities	58,899,000	59,704,000
Insurance investment contract
Available-for-sale securities [Abstract]
Available-for-sale securities, amortized cost basis	37,250,000	31,884,000
Available-for-sale securities, gross unrealized gains	11,648,000	6,468,000
Available-for-sale securities, gross unrealized losses	0	0
Available-for-sale securities	48,898,000	38,352,000
Auction rate securities
Available-for-sale securities [Abstract]
Available-for-sale securities, amortized cost basis		11,400,000
Available-for-sale securities, gross unrealized gains		0
Available-for-sale securities, gross unrealized losses		0
Available-for-sale securities		11,400,000
Mortgage-backed securities
Available-for-sale securities [Abstract]
Available-for-sale securities, amortized cost basis	8,054,000	8,206,000
Available-for-sale securities, gross unrealized gains	144,000	95,000
Available-for-sale securities, gross unrealized losses	(3,000)	(5,000)
Available-for-sale securities	8,195,000	8,296,000
U.S. Treasury securities
Available-for-sale securities [Abstract]
Available-for-sale securities, amortized cost basis	1,763,000	1,619,000
Available-for-sale securities, gross unrealized gains	43,000	37,000
Available-for-sale securities, gross unrealized losses	0	0
Available-for-sale securities	$ 1,806,000	$ 1,656,000

Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

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Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

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Reference 3: http://www.xbrl.org/2003/role/presentationRef

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Reference 4: http://www.xbrl.org/2003/role/presentationRef

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Reference 5: http://www.xbrl.org/2003/role/presentationRef

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Reference 6: http://www.xbrl.org/2003/role/presentationRef

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-Publisher FASB

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Reference 9: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 115

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Reference 10: http://www.xbrl.org/2003/role/presentationRef

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+ Details

- Definition

This item represents the cost of debt and equity securities, which are categorized neither as held-to-maturity nor trading, net of adjustments including accretion, amortization, collection of cash, previous other-than-temporary impairments recognized in earnings (less any cumulative-effect adjustments recognized, as defined), and fair value hedge accounting adjustments, if any.

+ References+ Details

- Definition

This item represents the gross unrealized gains for securities, at a point in time, which are categorized neither as held-to-maturity nor trading securities.

+ References+ Details