Reconciliation of Weighted Average Shares Used in Computation of Earnings Per Share (Detail) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Earnings Per Share Disclosure [Line Items]
Net income available to common stockholders	$ 30,612	$ 45,845	$ 8,828	$ 32,584
Effect of dilutive securities, convertible senior notes	1,840	1,779	5,557	5,516
Net income available to common stockholders plus assumed exercises and conversion	$ 32,452	$ 47,624	$ 14,385	$ 38,100
Earnings per share - basic, Weighted Average Shares
Weighted Average Shares, available to common stockholders	16,044	18,559	16,561	18,929
Effect of dilutive securities:
Convertible senior notes	23,081	23,369	23,123	23,369
Earnings per share - diluted, Weighted Average Shares
Net income available to common stockholders plus assumed exercises and conversion	39,504	42,562	39,916	42,737
Earnings per share - basic
Net income available to common stockholders	$ 1.91	$ 2.47	$ 0.53	$ 1.72
Earnings per share - diluted
Net income available to common stockholders plus assumed exercises and conversion	$ 0.82	$ 1.12	$ 0.36	$ 0.89
Performance Stock
Earnings Per Share Disclosure [Line Items]
Effect of dilutive securities, unvested stock grants	$ 0	$ 0	$ 0	$ 0
Effect of dilutive securities:
Unvested stock grants	164	409	115	284
Restricted Stock
Earnings Per Share Disclosure [Line Items]
Effect of dilutive securities, unvested stock grants	$ 0	$ 0	$ 0	$ 0
Effect of dilutive securities:
Unvested stock grants	215	225	117	155