Subsequent events (Details) (Subsequent Event [Member], Employee Severance [Member], Workforce Reduction Plan [Member], USD $) In Thousands, unless otherwise specified	0 Months Ended	3 Months Ended
	Feb. 02, 2015 employee	Mar. 31, 2015
Subsequent Event [Line Items]
Number of Positions Eliminated	180
Expected costs	$ 9,994
Restructuring liability	3,356
Scenario, Forecast [Member]
Subsequent Event [Line Items]
Restructuring expenses		$ 6,638
Various Field Offices [Member]
Subsequent Event [Line Items]
Number of Positions Eliminated	59
Oklahoma City [Member] | Headquarters Office [Member]
Subsequent Event [Line Items]
Number of Positions Eliminated	121