Acquisitions and Disposals - Transaction Related Costs - Catlin (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2016 USD ($)
Business Acquisition [Line Items]
Liabilities at December 31, 2015	$ 57,289
Costs incurred in 2016	220,355
2016 payments	240,678
Liabilities at December 31, 2016	36,966
Severance related costs
Business Acquisition [Line Items]
Liabilities at December 31, 2015	16,127
Costs incurred in 2016	57,098
2016 payments	47,865
Liabilities at December 31, 2016	25,360
Retention and other compensation costs
Business Acquisition [Line Items]
Liabilities at December 31, 2015	16,969
Costs incurred in 2016	33,220
2016 payments	45,708
Liabilities at December 31, 2016	4,481
Facilities-related costs
Business Acquisition [Line Items]
Liabilities at December 31, 2015	818
Costs incurred in 2016	30,236
2016 payments	31,036
Liabilities at December 31, 2016	18
Consulting and other
Business Acquisition [Line Items]
Liabilities at December 31, 2015	23,375
Costs incurred in 2016	99,801
2016 payments	116,069
Liabilities at December 31, 2016	$ 7,107