Selected Quarterly Financial Data (unaudited) (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Quarterly Financial Information Disclosure [Abstract]
Revenues	$ 150,540	$ 162,842	$ 237,509	$ 182,115	$ 197,179	$ 198,899	$ 159,404	$ 195,110	$ 733,006	$ 750,592	$ 711,723
Gross profit	47,927	46,849	58,670	45,894	49,797	42,787	43,534	42,494	199,340	178,612	148,351
Net earnings	$ 22,723	$ 23,824	$ 32,216	$ 20,770	$ 24,370	$ 20,970	$ 23,619	$ 17,937	$ 99,533	$ 86,896	$ 63,823
Net earnings per common share—basic and diluted (in dollars per share)	$ 1.06	$ 1.12	$ 1.51	$ 0.97	$ 1.14	$ 0.98	$ 1.11	$ 0.84	$ 4.66	$ 4.07	$ 2.99