Investments - Additional Information (Details) $ in Thousands	Dec. 31, 2015 USD ($) security	Dec. 31, 2014 USD ($) security	Oct. 31, 2013 USD ($)
Investment Holdings [Line Items]
Investments at cost	$ 196,402	$ 167,740
Gain on sale of asset-backed security			$ 63,800
Par value of mortgage-backed securities pledged to FRB	188,300	160,900
Mortgage-backed securities
Investment Holdings [Line Items]
Investments at cost	$ 196,402	$ 167,740
Number of mortgage-backed securities with unrealized losses | security	35	13
Number of mortgage-backed securities | security	74	56
Ginnie Mae | Mortgage-backed securities
Investment Holdings [Line Items]
Investments at cost	$ 93,600
Number of mortgage-backed securities with unrealized losses | security	14	9
Number of mortgage-backed securities | security	35
Fannie Mae | Mortgage-backed securities
Investment Holdings [Line Items]
Investments at cost	$ 78,700
Freddie Mac | Mortgage-backed securities
Investment Holdings [Line Items]
Investments at cost	$ 24,100