Earnings Per Share Reconciliation (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
NUMERATOR
NET INCOME	$ 42,602	$ 32,042	$ 69,059	$ 55,569
Less: undistributed earnings allocated to nonvested restricted stock	0	0	0	(35)
Numerator for basic earnings per share	42,602	32,042	69,059	55,534
Plus: undistributed earnings allocated to nonvested restricted stock	0	0	0	35
Less: undistributed earnings reallocated to nonvested restricted stock	0	0	0	(28)
Numerator for diluted earnings per share	$ 43,630	$ 33,070	$ 71,115	$ 57,597
DENOMINATOR
Basic earnings per share—weighted-average shares	46,706,437	46,914,902	46,579,660	46,747,403
Effect of dilutive securities:
Share-based payments (in shares)	977,421	926,184	1,004,661	975,081
Diluted earnings per share-adjusted weighted-average shares and assumed conversions	58,273,600	58,430,828	58,174,063	58,312,226
NET INCOME PER COMMON SHARE
Basic (in dollars per share)	$ 0.91	$ 0.68	$ 1.48	$ 1.19
Diluted (in dollars per share)	$ 0.75	$ 0.57	$ 1.22	$ 0.99
1.6 percent convertible senior notes due May 2018
NUMERATOR
Plus: interest on convertible senior notes	$ 729	$ 729	$ 1,458	$ 1,458
Effect of dilutive securities:
Convertible senior notes (in shares)	7,023,780	7,023,780	7,023,780	7,023,780
NET INCOME PER COMMON SHARE
Interest rate stated percentage	1.60%		1.60%
0.25 percent convertible senior notes due June 2019
NUMERATOR
Plus: interest on convertible senior notes	$ 299	$ 299	$ 598	$ 598
Effect of dilutive securities:
Convertible senior notes (in shares)	3,565,962	3,565,962	3,565,962	3,565,962
NET INCOME PER COMMON SHARE
Interest rate stated percentage	0.25%		0.25%