Investments Pledged assets and net payable receivable for securities purchased sold (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Pledged assets and net payable for securities purchased [Line Items]
Funds Held Investment Interest Rate Range Low	0.10%	2.10%	1.80%
Funds Held Investment Interest Rate Range High	8.00%	5.40%	4.30%
Restricted Cash and Cash Equivalents	$ 164,800	$ 172,300
Marketable Securities, Restricted	2,168,300	2,455,600
Receivable for securities sold	34,497	51,586
Payable for securities purchased	(219,707)	(63,779)
Net payable for securities purchased	$ (185,210)	$ (12,193)