Condensed Consolidated Statements Of Shareholders' Equity (USD $) In Thousands, except Share data, unless otherwise specified	Total	Common Stock [Member]	Retained Earnings [Member]	Accumulated Other Comprehensive Gain (Loss) [Member]
Balances at Dec. 31, 2013	$ 399,364	$ 30,386	$ 372,231	$ (3,253)
Balances, shares at Dec. 31, 2013	60,137,758	60,137,758
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Stock compensation for equity incentives and restricted common stock granted to employees	17,469	17,469
Stock compensation for equity incentives and restricted common stock granted to employees, shares		576,178
Issuance of common stock pursuant to employee stock purchase plan	2,424	2,424
Issuance of common stock pursuant to employee stock purchase plan, shares		46,866
Dividends declared on shares of common stock	(36,790)		(36,790)
Issuance of common stock upon exercise of stock options	11,716	11,716
Issuance of common stock upon exercise of stock options, shares		872,954
Repurchase of common stock	(2,146)	(2,146)	0
Repurchase of common stock, shares		(23,552)
Cancellation of shares related to tax liability, value	(12,621)	(12,621)
Cancellation of shares related to tax liability, shares		(161,267)
Income tax benefit realized from share-based compensation plans	17,699	17,699
Comprehensive income (loss):
Net unrealized gain on investments	37			37
Foreign currency translation adjustments	242			242
Net income	170,746		170,746
Balances at Jun. 30, 2014	$ 568,140	$ 64,927	$ 506,187	$ (2,974)
Balances, shares at Jun. 30, 2014	61,448,937	61,448,937