Acquisitions and Disposals - Allied Acquisition (Details) - USD ($) $ in Thousands		12 Months Ended
	Feb. 01, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Apr. 11, 2017
Business Acquisition [Line Items]
Indefinite-lived Intangible Assets Acquired			$ 8,000	$ 673,000
Finite-lived Intangible Assets Acquired			28,948	315,000
Goodwill, acquired during period			$ 14,084	$ 807,381
Allied International Holdings, Inc.
Business Acquisition [Line Items]
Cash consideration	$ 75,700
Contingent Consideration Measurement Period	3 years
Business Combination, Contingent Consideration Arrangements, Range of Outcomes, Value, Low					$ 12,500
Business Combination, Contingent Consideration Arrangements, Range of Outcomes, Value, High					$ 20,000
Business Combination, Contingent Consideration, Liability	$ 15,000	$ 17,000
Change in Estimate of Contingent Consideration		$ 2,000
Acquisition consideration	90,700
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Net	76,700
Indefinite-lived Intangible Assets Acquired	8,000
Finite-lived Intangible Assets Acquired	6,000
Goodwill, acquired during period	14,100
Insurance Segment | Allied International Holdings, Inc.
Business Acquisition [Line Items]
Goodwill, acquired during period	$ 14,100