Fair value - other postretirement plans (Details 11) (USD $)	Dec. 31, 2012		Dec. 31, 2011		Dec. 31, 2010
Defined Benefit Plan Disclosure [Line Items]
Percentage investment in common stock of large-cap U.S. companies	29.00%		32.00%
Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Percentage investment in common stock of large-cap U.S. companies	51.00%
Percentage investment in mortgage backed securities	10.00%		12.00%
Percentage investment in common stock of large-cap U.S. companies			49.00%
Percentage investment in US Treasuries	15.00%		15.00%
Percentage investment in corporate bonds	11.00%		11.00%
Percentage investment in other investments	13.00%		13.00%
Fair value of net plan assets at end of year	$ 74,361,000		$ 68,085,000		$ 70,610,000
Estimated future employer contributions in next fiscal year	2,300,000
Other Postretirement Benefits | Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Fair value of net plan assets at end of year	70,841,000		65,666,000
Other Postretirement Benefits | Significant Unobservable Inputs (Level 3)
Defined Benefit Plan Disclosure [Line Items]
Fair value of net plan assets at end of year	0		0
Other Postretirement Benefits | Quoted Prices in Active Markets for Identical Assets (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Fair value of net plan assets at end of year	3,520,000		2,419,000
Other Postretirement Benefits | Cash equivalents
Defined Benefit Plan Disclosure [Line Items]
Fair value of net plan assets at end of year	3,044,000		1,895,000
Other Postretirement Benefits | Cash equivalents | Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Fair value of net plan assets at end of year	1,991,000		1,836,000
Other Postretirement Benefits | Cash equivalents | Significant Unobservable Inputs (Level 3)
Defined Benefit Plan Disclosure [Line Items]
Fair value of net plan assets at end of year	0		0
Other Postretirement Benefits | Cash equivalents | Quoted Prices in Active Markets for Identical Assets (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Fair value of net plan assets at end of year	1,053,000		59,000
Other Postretirement Benefits | Equity securities of U.S. companies
Defined Benefit Plan Disclosure [Line Items]
Fair value of net plan assets at end of year	2,207,000		2,098,000
Other Postretirement Benefits | Equity securities of U.S. companies | Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Fair value of net plan assets at end of year	0		0
Other Postretirement Benefits | Equity securities of U.S. companies | Significant Unobservable Inputs (Level 3)
Defined Benefit Plan Disclosure [Line Items]
Fair value of net plan assets at end of year	0		0
Other Postretirement Benefits | Equity securities of U.S. companies | Quoted Prices in Active Markets for Identical Assets (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Fair value of net plan assets at end of year	2,207,000		2,098,000
Other Postretirement Benefits | Equity securities of International companies
Defined Benefit Plan Disclosure [Line Items]
Fair value of net plan assets at end of year	260,000		262,000
Other Postretirement Benefits | Equity securities of International companies | Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Fair value of net plan assets at end of year	0		0
Other Postretirement Benefits | Equity securities of International companies | Significant Unobservable Inputs (Level 3)
Defined Benefit Plan Disclosure [Line Items]
Fair value of net plan assets at end of year	0		0
Other Postretirement Benefits | Equity securities of International companies | Quoted Prices in Active Markets for Identical Assets (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Fair value of net plan assets at end of year	260,000		262,000
Other Postretirement Benefits | Insurance investment contract
Defined Benefit Plan Disclosure [Line Items]
Fair value of net plan assets at end of year	68,850,000	[1]	63,830,000	[2]
Other Postretirement Benefits | Insurance investment contract | Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Fair value of net plan assets at end of year	68,850,000	[1]	63,830,000	[2]
Other Postretirement Benefits | Insurance investment contract | Significant Unobservable Inputs (Level 3)
Defined Benefit Plan Disclosure [Line Items]
Fair value of net plan assets at end of year	0	[1]	0	[2]
Other Postretirement Benefits | Insurance investment contract | Quoted Prices in Active Markets for Identical Assets (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Fair value of net plan assets at end of year	$ 0	[1]	$ 0	[2]

[1] The insurance investment contract invests approximately 51 percent in common stock of large-cap U.S. companies, 15 percent in U.S. Treasuries, 10 percent in mortgage-backed securities, 11 percent in corporate bonds and 13 percent in other investments.

[2] The insurance investment contract invests approximately 49 percent in common stock of large-cap U.S. companies, 15 percent in U.S. Treasuries, 12 percent in mortgage-backed securities, 11 percent in corporate bonds and 13 percent in other investments.