Summary of Investments - SOI Ltd partnership Sec lending Special deposits (Details) - USD ($)	Mar. 31, 2018	Dec. 31, 2017
Nature and activities of the VIEs and the effect on financial statements
Carrying value to loss in relation to the activities of VIEs	$ 53,120,000	$ 45,540,000
Maximum exposure to loss in relation to the activities of the VIEs	53,120,000	45,540,000
Special deposits and securities lending
Amortized cost	128,151,000
Securities loaned to third parties at fair value	122,278,000	0
Collateral received	95,024,000	0
Collateral fair value	31,424,000
Payable under securities lending agreements	95,024,000	$ 0
Corporate debt securities
Special deposits and securities lending
Payable under securities lending agreements	$ 95,024,000