Available-for-sale securities (Details) (USD $)	3 Months Ended
Mar. 31, 2013	Mar. 31, 2012	Dec. 31, 2012
Schedule of available-for-sale securities [Line Items]
Investments used to satisfy obligations under nonqualified benefit plans	$ 53,300,000	$ 48,700,000	$ 48,900,000
Net unrealized gain (loss) of investments used to satify obligations under nonqualified benefit plans	4,400,000	5,000,000
Available-for-sale securities [Abstract]
Available-for-sale securities, amortized cost basis	47,320,000	58,570,000	47,067,000
Available-for-sale securities, gross unrealized gains	16,236,000	11,622,000	11,835,000
Available-for-sale securities, gross unrealized losses	(3,000)	(2,000)	(3,000)
Available-for-sale securities	63,553,000	70,190,000	58,899,000
Insurance contract [Member]
Available-for-sale securities [Abstract]
Available-for-sale securities, amortized cost basis	37,270,000	37,250,000	37,250,000
Available-for-sale securities, gross unrealized gains	16,064,000	11,454,000	11,648,000
Available-for-sale securities, gross unrealized losses	0	0	0
Available-for-sale securities	53,334,000	48,704,000	48,898,000
Auction rate securities [Member]
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
Collateralized mortgage backed securities [Member]
Available-for-sale securities [Abstract]
Available-for-sale securities, amortized cost basis	8,749,000	7,952,000	8,054,000
Available-for-sale securities, gross unrealized gains	133,000	119,000	144,000
Available-for-sale securities, gross unrealized losses	(3,000)	(1,000)	(3,000)
Available-for-sale securities	8,879,000	8,070,000	8,195,000
US Treasury securities [Member]
Available-for-sale securities [Abstract]
Available-for-sale securities, amortized cost basis	1,301,000	1,968,000	1,763,000
Available-for-sale securities, gross unrealized gains	39,000	49,000	43,000
Available-for-sale securities, gross unrealized losses	0	(1,000)	0
Available-for-sale securities	$ 1,340,000	$ 2,016,000	$ 1,806,000

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

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

-Publisher FASB

-Name FASB Staff Position (FSP)

-Number FAS115-1/124-1

-Paragraph 15D

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009.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 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 115

-Paragraph 12

-Subparagraph b

-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 Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 45

-Paragraph 1

-URI http://asc.fasb.org/extlink&oid=6871852&loc=d3e26610-111562

Reference 4: 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 5: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name FASB Staff Position (FSP)

-Number FAS115-1/124-1

-Paragraph 15C

-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 Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 25

-Paragraph 1

-Subparagraph (b)

-URI http://asc.fasb.org/extlink&oid=7534914&loc=d3e22054-111558

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

-Publisher FASB

-Name FASB Staff Position (FSP)

-Number FAS115-1/124-1

-Paragraph 15E

-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 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 9: 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 10: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 133

-Paragraph 22

-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.

+ 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