Consolidated Statements of Shareholders' Equity (USD $) In Thousands, unless otherwise specified	Total	Common Stock	Additional Paid-In Capital	Retained Earnings	Accumulated Other Comprehensive Income (Loss)	Treasury Stock	Noncontrolling Interest
Balance at beginning of period at Dec. 31, 2013	$ 1,333,327	$ 2,019	$ 642,164	$ 455,632	$ (2,383)	$ 0	$ 235,895
Balance at beginning of period (Shares) at Dec. 31, 2013		153,524
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	60,362			41,863			18,499
Foreign currency translation adjustments	(57)				(42)		(15)
Unrealized loss on marketable securities	372				277		95
Stock-based compensation expense	4,889		4,889
Distribution to noncontrolling interest	(4,675)			0			(4,675)
Common stock dividends	(11,514)		0	(11,514)
Balance at end of period at Mar. 31, 2014	1,381,960	2,019	647,053	485,981	(2,702)	0	249,609
Balance at end of period (Shares) at Mar. 31, 2014		153,524
Balance at beginning of period at Dec. 31, 2014	1,472,536	2,033	683,611	545,357	(14,210)	(4,801)	260,546
Balance at beginning of period (Shares) at Dec. 31, 2014		154,327
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	46,401			34,279			12,122
Foreign currency translation adjustments	(11,747)				(8,749)		(2,998)
Unrealized loss on marketable securities	(354)				(264)		(90)
Stock-based compensation expense	8,010		8,010
Amount withheld for employee stock purchase plan	55		55
Distribution to noncontrolling interest	(20,982)						(20,982)
Common stock dividends	(23,150)			(23,150)
Common shares issued upon vesting of restricted stock units (Shares)		6
Common shares issued upon vesting of restricted stock units		0	0
Treasury shares withheld (Shares)		(2)
Treasury shares withheld	(36)					(36)
Balance at end of period at Mar. 31, 2015	$ 1,471,441	$ 2,033	$ 691,676	$ 556,486	$ (22,695)	$ (4,837)	$ 248,778
Balance at end of period (Shares) at Mar. 31, 2015		154,331