Consolidated Statements of Comprehensive Income - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Statement of Comprehensive Income [Abstract]
Net income	$ 469,053	$ 431,620	$ 450,003
Foreign currency translation	(70,481)	(61,728)	11,843
Unrealized (losses) gains on investments before reclassifications	(1,825)	(532)	134
Amounts reclassified to other expense for unrealized (losses) gains on investments			1,575
Unrealized (losses) gains on investments before income taxes	(1,825)	(532)	1,709
Income tax benefit (expense) on unrealized (losses) gains on investments	(31)	(43)	639
Unrealized (losses) gains on investments, net of tax	(1,794)	(489)	1,070
Retirement liability adjustment before reclassifications	191	(34,797)	27,888
Retirement liability amounts reclassified to selling and administrative expenses	4,443	2,886	3,678
Retirement liability adjustment	4,634	(31,911)	31,566
Income tax (expense) benefit on retirement liability adjustment	(380)	9,544	(12,205)
Retirement liability adjustment, net of tax	4,254	(22,367)	19,361
Other comprehensive (loss) income	(68,021)	(84,584)	32,274
Comprehensive income	$ 401,032	$ 347,036	$ 482,277