Nonqualified benefit plans (Details 13) (USD $)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Supplemental employee retirement plans, defined benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Investments in nonqualified benefit plans	$ 84,400,000	$ 76,900,000
Net periodic benefit cost	8,100,000	8,100,000	7,800,000
Projected benefit obligation	113,000,000	113,800,000
Accumulated benefit obligation	107,500,000	105,700,000
Discount rate	3.44%	4.00%
Rate of compensation increase	3.00%	4.00%
Discount rate	4.00%	5.11%
Rate of compensation increase	4.00%	4.00%
Expected future benefit payments, next twelve months	5,700,000
Expected future benefit payments, year two	5,600,000
Expected future benefit payments, year three	6,700,000
Expected future benefit payments, year four	6,500,000
Expected future benefit payments, year five	6,700,000
Expected future benefit payments, five fiscal years thereafter	37,300,000
Supplemental employee retirement plans, defined benefit [Member] | Equity Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Investments in nonqualified benefit plans	41,900,000	38,400,000
Supplemental employee retirement plans, defined benefit [Member] | Life insurance carried on plan participants [Member]
Defined Benefit Plan Disclosure [Line Items]
Investments in nonqualified benefit plans	32,700,000	31,800,000
Supplemental employee retirement plans, defined benefit [Member] | Other Investments [Member]
Defined Benefit Plan Disclosure [Line Items]
Investments in nonqualified benefit plans	9,800,000	6,700,000
Supplemental employee retirement plans defined contribution plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Plan, cost recognized	$ 84,000