Earnings Per Common Share (Details) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended			6 Months Ended	9 Months Ended	12 Months Ended
	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Jun. 30, 2014	Sep. 30, 2014	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Earnings Per Share [Abstract]
Net earnings	$ 25,925	$ 17,681	$ 27,266	$ 44,947	$ 70,872	$ 96,422	$ 128,757	$ 180,018
Basic weighted average shares						52,850,000	54,804,000	58,913,000
Effect of dilutive stock options						276,000	358,000	492,000
Diluted weighted average shares						53,126,000	55,162,000	59,405,000
Basic earnings per common share	$ 0.49	$ 0.33	$ 0.52	$ 0.85	$ 1.34	$ 1.82	$ 2.35	$ 3.06
Diluted earnings per common share	$ 0.49	$ 0.33	$ 0.51	$ 0.84	$ 1.33	$ 1.81	$ 2.33	$ 3.03
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Number of anti-dilutive stock options						2,496,147	1,507,355	1,115,245