Nonqualified benefit plans (Details 12) (USD $)	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Supplemental employee retirement plan, defined benefit [Member]
Defined Benefit and Defined Contribution Plans Disclosure [Line Items]
Net periodic benefit cost	$ 7,300,000	$ 8,100,000	$ 8,100,000
Projected benefit obligation	106,900,000	113,000,000
Accumulated benefit obligation	99,700,000	107,500,000
Discount rate	4.32%	3.44%
Rate of compensation increase	4.00%	3.00%
Discount rate	3.44%	4.00%
Rate of compensation increase	3.00%	4.00%
Expected future benefit payments, next twelve months	5,700,000
Expected future benefit payments, year two	6,700,000
Expected future benefit payments, year three	6,500,000
Expected future benefit payments, year four	6,700,000
Expected future benefit payments, year five	7,200,000
Expected future benefit payments, five fiscal years thereafter	37,500,000
Supplemental employee retirement plan, defined contribution [Member]
Defined Benefit and Defined Contribution Plans Disclosure [Line Items]
Plan, cost recognized	304,000	84,000
Supplemental employee retirement plans, defined contribution and defined benefit plans [Member]
Defined Benefit and Defined Contribution Plans Disclosure [Line Items]
Investments in nonqualified benefit plans	98,100,000	84,400,000
Supplemental employee retirement plans, defined contribution and defined benefit plans [Member] | Equity securities [Member]
Defined Benefit and Defined Contribution Plans Disclosure [Line Items]
Investments in nonqualified benefit plans	53,500,000	41,900,000
Supplemental employee retirement plans, defined contribution and defined benefit plans [Member] | Life insurance carried on plan participants [Member]
Defined Benefit and Defined Contribution Plans Disclosure [Line Items]
Investments in nonqualified benefit plans	31,400,000	32,700,000
Supplemental employee retirement plans, defined contribution and defined benefit plans [Member] | Other investments [Member]
Defined Benefit and Defined Contribution Plans Disclosure [Line Items]
Investments in nonqualified benefit plans	$ 13,200,000	$ 9,800,000