Available-for-sale securities (Details) (USD $)	3 Months Ended
Mar. 31, 2012	Mar. 31, 2011	Dec. 31, 2011
Schedule of available-for-sale securities [Line Items]
Investments used to satisfy obligations under nonqualified benefit plans	$ 48,700,000	$ 41,600,000	$ 38,400,000
Before tax amount of increase (decrease) in fair value of investments used to satisfy obligations under nonqualified benefit plans	5,000,000	2,100,000
Available-for-sale securities [Abstract]
Available-for-sale securities, amortized cost basis	58,570,000		53,109,000
Available-for-sale securities, gross unrealized gains	11,622,000		6,600,000
Available-for-sale securities, gross unrealized losses	(2,000)		(5,000)
Available-for-sale securities	70,190,000		59,704,000
Insurance investment contract [Member]
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	11,454,000		6,468,000
Available-for-sale securities, gross unrealized losses	0		0
Available-for-sale securities	48,704,000		38,352,000
Auction rate securities [Member]
Available-for-sale securities [Abstract]
Available-for-sale securities, amortized cost basis	11,400,000		11,400,000
Available-for-sale securities, gross unrealized gains	0		0
Available-for-sale securities, gross unrealized losses	0		0
Available-for-sale securities	11,400,000		11,400,000
Collateralized mortgage backed securities [Member]
Available-for-sale securities [Abstract]
Available-for-sale securities, amortized cost basis	7,952,000		8,206,000
Available-for-sale securities, gross unrealized gains	119,000		95,000
Available-for-sale securities, gross unrealized losses	(1,000)		(5,000)
Available-for-sale securities	8,070,000		8,296,000
US Treasury securities [Member]
Available-for-sale securities [Abstract]
Available-for-sale securities, amortized cost basis	1,968,000		1,619,000
Available-for-sale securities, gross unrealized gains	49,000		37,000
Available-for-sale securities, gross unrealized losses	(1,000)		0
Available-for-sale securities	$ 2,016,000		$ 1,656,000

For an unclassified balance sheet, this item represents investments in debt and equity securities which are categorized neither as held-to-maturity nor trading.

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

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

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-Name Statement of Financial Accounting Standard (FAS)

-Number 115

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-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

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

-Publisher FASB

-Name FASB Staff Position (FSP)

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-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

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

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

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-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

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

-Publisher FASB

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-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

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

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 115

-Paragraph 13

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 7: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name FASB Staff Position (FSP)

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-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 8: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 115

-Paragraph 14

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

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

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 115

-Paragraph 16

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

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

-Publisher FASB

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-Section 25

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-URI http://asc.fasb.org/extlink&oid=6379932&loc=d3e22054-111558

+ 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