Consolidated Statements of Changes in Equity - USD ($) $ in Thousands	Common Stock	Additional Paid-In Capital	Accumulated Other Comprehensive Income (Loss)	Treasury Stock	Retained Earnings	Redeemable Noncontrolling Interest	Total
Balance at Dec. 31, 2017						$ 115,689
Changes in Redeemable Noncontrolling Interests
Net income						1,261
Adjustments to redemption value of redeemable noncontrolling interest						9,051
Subsidiary dividends paid to noncontrolling interests						(1)
Balance at Mar. 31, 2018						126,000
Balance at Dec. 31, 2017	$ 20,277	$ 162,806	$ (36,148)	$ (909,960)	$ 3,004,018		$ 2,240,993
Balance (in shares) at Dec. 31, 2017	202,765
Changes in Total Equity
Net income					141,841		141,841
Other comprehensive income			14,929				14,929
Common stock issued from treasury shares for exercise of stock options		5,199		21,258			26,457
Common stock issued from treasury shares for nonvested awards		(551)		551
Common stock unissued due to forfeiture of nonvested awards		551		(551)
Common stock issued from treasury shares for nonvested awards		(12,368)		12,368
Common stock issued from treasury for dividend equivalents		925		9			934
Share-based compensation (Note 7)		6,835					6,835
Cash dividends declared ($0.13 per share)					(23,895)		(23,895)
Purchase of treasury shares				(24)			(24)
Adjustments to redemption value of redeemable noncontrolling interest		(9,051)					(9,051)
Balance at Mar. 31, 2018	$ 20,277	154,897	(21,219)	(876,900)	3,117,519		2,394,574
Balance (in shares) at Mar. 31, 2018	202,765
Balance at Dec. 31, 2017						$ 115,689
Balance at Dec. 31, 2017	$ 20,277	162,806	(36,148)	(909,960)	3,004,018		2,240,993
Balance (in shares) at Dec. 31, 2017	202,765
Changes in Total Equity
Reclassification from adoption of ASU 2018-02			(5,052)
Balance at Dec. 31, 2018	$ 20,277	189,889	(60,223)	(1,042,687)	3,478,650		$ 2,585,906
Balance (in shares) at Dec. 31, 2018	202,765						202,765,000
Changes in Total Equity
Cumulative effect adjustment from adoption of ASU | ASU 2014-09					(4,445)		$ (4,445)
Net income					161,607		161,607
Other comprehensive income			10,247				10,247
Common stock issued from treasury shares for exercise of stock options		3,485		2,981			6,466
Common stock issued from treasury shares for nonvested awards		77		(77)
Common stock unissued due to forfeiture of nonvested awards		(77)		77
Common stock issued from treasury shares for nonvested awards		(12,812)		12,812
Share-based compensation (Note 7)		11,296					11,296
Cash dividends declared ($0.13 per share)					(22,546)		(22,546)
Purchase of treasury shares				(400,013)			(400,013)
Balance at Mar. 31, 2019	$ 20,277	$ 191,935	(55,028)	$ (1,426,984)	3,622,560		$ 2,352,760
Balance (in shares) at Mar. 31, 2019	202,765						202,765,000
Changes in Total Equity
Cumulative effect adjustment from adoption of ASU | Adjustments for New Accounting Pronouncement | ASU 2016-01			$ (5,052)		5,052
Cumulative effect adjustment from adoption of ASU | Adjustments for New Accounting Pronouncement | ASU 2016-02					$ (203)		$ (203)