Retirement Plans - Narrative (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2017 USD ($) funds	Dec. 31, 2016 USD ($) funds	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)
Defined Benefit Plan Disclosure [Line Items]
Vesting period for defined contribution plan	5 years
Expense recognized on defined contribution plans	$ 73,500	$ 88,300	$ 87,500
Number of years of compensation for calculating benefit	3 years
Number of years of employment considered at retirement	10 years
Defined benefit plan
Defined Benefit Plan Disclosure [Line Items]
Funded status - end of year	$ (38,578)	(42,786)	(42,763)
Fair value of plan assets	85,848	77,252	79,402	$ 49,281
Accrued pension liability	38,578	42,786	$ 42,763
Defined benefit plan | United States
Defined Benefit Plan Disclosure [Line Items]
Funded status - end of year	(13,700)	(14,093)
Projected benefit obligation	$ 2,400	$ 2,300
Number of mutual funds held by plan | funds	3	3
Fair value of plan assets	$ 42,700	$ 38,700
Estimated employer contributions in next fiscal year	$ 1,400