Fair Value of Assets and Liabilities - Not Measured at Fair Value (Details) - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets	$ 4,414,742	$ 2,800,686
Total liabilities	4,099,875	2,817,591
Level 1 Inputs
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets	0	0
Total liabilities	0	0
Level 2 Inputs
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets	338,519	0
Total liabilities	0	0
Level 3 Inputs
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets	4,076,223	2,800,686
Total liabilities	4,099,875	2,817,591
Carrying Amount
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	579,206	869,780
Restricted cash	159,216	46,763
Deposits	763	657
Total assets	739,185	917,200
Accounts payable	3,756	5,892
Payables to investors	150,880	38,741
Total liabilities	154,636	44,633
Portion at Other than Fair Value Measurement | Level 1 Inputs
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	0	0
Restricted cash	0	0
Deposits	0	0
Total assets	0	0
Accounts payable	0	0
Payables to investors	0	0
Total liabilities	0	0
Portion at Other than Fair Value Measurement | Level 2 Inputs
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	579,206	869,780
Restricted cash	159,216	46,763
Deposits	763	657
Total assets	739,185	917,200
Accounts payable	3,756	5,892
Payables to investors	150,880	38,741
Total liabilities	154,636	44,633
Portion at Other than Fair Value Measurement | Level 3 Inputs
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	0	0
Restricted cash	0	0
Deposits	0	0
Total assets	0	0
Accounts payable	0	0
Payables to investors	0	0
Total liabilities	0	0
Balance at Fair Value
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	579,206	869,780
Restricted cash	159,216	46,763
Deposits	763	657
Total assets	739,185	917,200
Accounts payable	3,756	5,892
Payables to investors	150,880	38,741
Total liabilities	$ 154,636	$ 44,633