1) For defined benefit pension plans, the benefit obligation is the projected benefit obligation, which is the actuarial present value as of a date of all benefits attributed by the pension benefit formula to employee service rendered prior to that date. 2) For other postretirement defined benefit plans, the benefit obligation is the accumulated postretirement benefit obligation, which is the actuarial present value of benefits attributed to employee service rendered to a particular date.

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Reference 2: http://www.xbrl.org/2003/role/presentationRef
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Reference 3: http://www.xbrl.org/2003/role/presentationRef
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Fair value of assets that have been segregated and restricted to provide pension or postretirement benefits. Assets include, but are not limited to, stocks, bonds, other investments, earnings from investments, and contributions by the employer and employees.

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Retirement Plans - Narrative (Details) $ in Thousands	12 Months Ended
Retirement Plans - Narrative (Details) $ in Thousands	Dec. 31, 2016 USD ($) funds	Dec. 31, 2015 USD ($) funds	Dec. 31, 2014 USD ($)	Jun. 30, 2015 USD ($)
Defined Benefit Plan Disclosure [Line Items]
Vesting period for defined contribution plan	5 years
Expense recognized on defined contribution plans	$ 88,300	$ 87,500	$ 56,600
Number of years of compensation for calculating benefit	3 years
Number of years of employment considered at retirement	10 years
Estimated employer contributions in next fiscal year	$ 1,400
US Plan
Defined Benefit Plan Disclosure [Line Items]
Funded status - end of year	$ (14,093)	$ (16,306)
Number of mutual funds held by plan \| funds	3	2
Fair value of plan assets	$ 38,700	$ 36,700
Benefits equalization plan
Defined Benefit Plan Disclosure [Line Items]
Projected benefit obligation	2,300	3,200
UK Plans
Defined Benefit Plan Disclosure [Line Items]
Funded status - end of year	(2,316)	(4,650)		$ (1,000)
Fair value of plan assets	$ 38,520	$ 42,716