Fair Value - narrative items (Details) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Fair Value - Other Details [Line Items]
Carrying amount of other invested assets not at fair value	$ 208,000
Fair Value Assets Level 2 To Level 1 Transfers Amount	0	$ 0
Cash and cash equivalents	1,577,097	1,313,468	$ 1,496,485	$ 1,121,705
Accrued investment income	141,672	158,737
Funds held directly managed [Member]
Fair Value - Other Details [Line Items]
Other net assets	70,700	88,300
Cash and cash equivalents	64,600	42,300
Accrued investment income	4,500	5,700
Trading securities [Member] | Other invested assets [Member]
Fair Value - Other Details [Line Items]
Carrying amount of other invested assets not at fair value	$ 208,300	$ 187,300