Employee benefit plans (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Defined contribution plan [Line Items]
Period of time for which defined contribution plan contributions were discontinued			3 years
Expense for profit sharing, cash bonus and 401(k) plans
Profit sharing/cash bonus plans	$ 25,561	$ 33,007	$ 16,361
401(k) plan	7,004	6,680	6,226
Deferred compensation plan
Maximum percentage of base salary employees can defer	100.00%
Maximum percentage of bonus employees can defer	50.00%
Deferred compensation plan liability	3,266	2,719
Investments in company-owned life insurance policies which fund deferred compensation plan	12,363	14,105
Voluntary employee beneficiary association (VEBA) trust
Contributions to VEBA trust	25,700	34,070	36,792
Amount by which liability for incurred but not reported medical claims exceeded prepaid balance in VEBA trust	2,154	549
First 1% of wages contributed by employee
Defined contribution plan [Line Items]
Employer matching contribution to 401(k) plan, percentage	100.00%
Next 5% of wages contributed by employee
Defined contribution plan [Line Items]
Employer matching contribution to 401(k) plan, percentage	50.00%
100% employer match
Defined contribution plan [Line Items]
Employee contribution to 401(k) plan receiving employer match, percent of wages	1.00%
50% employer match
Defined contribution plan [Line Items]
Employee contribution to 401(k) plan receiving employer match, percent of wages	5.00%
Employees under the age of 50
Defined contribution plan [Line Items]
401(k) plan, maximum annual employee contribution, amount	18
401(k) plan, maximum annual employee contribution, percent of wages	50.00%
Employees aged 50 or older
Defined contribution plan [Line Items]
401(k) plan, maximum annual employee contribution, amount	$ 23