Investments - Narrative (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Sep. 30, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Schedule of Available-for-sale Securities [Line Items]
Realized gains (losses) on sale of securities		$ 5,311	$ (833)	$ 93
Proceeds from sales of securities		385,695	83,239	7,725
Maturities of securities		410,000	150,000
Available-for-sale securities, continuous unrealized loss position, less than twelve months, fair value		191,240	219,512
Available-for-sale securities, continuous unrealized loss position, less than twelve months, unrealized loss		1,117	1,310
Available-for-sale securities, continuous unrealized loss position, twelve months or longer, fair value		0	14,260
Available-for-sale securities, continuous unrealized loss position, 12 months or longer, accumulated loss		0	188
Gross Unrealized Losses		1,117	1,487
Other-than-temporary impairment losses	$ 1,300	0	1,295	$ 0
U.S. Treasury securities
Schedule of Available-for-sale Securities [Line Items]
U.S. Treasury securities		184,700
Available-for-sale securities, continuous unrealized loss position, less than twelve months, fair value		184,740	109,984
Available-for-sale securities, continuous unrealized loss position, less than twelve months, unrealized loss		79	6
Available-for-sale securities, continuous unrealized loss position, twelve months or longer, fair value		0	0
Available-for-sale securities, continuous unrealized loss position, 12 months or longer, accumulated loss		0	0
Gross Unrealized Losses		79	0
Corporate debt securities
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, continuous unrealized loss position, less than twelve months, fair value		6,292	85,845
Available-for-sale securities, continuous unrealized loss position, less than twelve months, unrealized loss		981	1,294
Available-for-sale securities, continuous unrealized loss position, twelve months or longer, fair value		0	14,260
Available-for-sale securities, continuous unrealized loss position, 12 months or longer, accumulated loss		0	188
Gross Unrealized Losses		981	1,482
Realized gain from the sale of available-for-sale securities		5,300
Preferred stock
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale securities, continuous unrealized loss position, less than twelve months, fair value		208	23,683
Available-for-sale securities, continuous unrealized loss position, less than twelve months, unrealized loss		57	10
Available-for-sale securities, continuous unrealized loss position, twelve months or longer, fair value		0	0
Available-for-sale securities, continuous unrealized loss position, 12 months or longer, accumulated loss		0	0
Gross Unrealized Losses		$ 57	$ 5
Cash, investment grade cash equivalents or U.S. treasury securities | Minimum
Schedule of Available-for-sale Securities [Line Items]
Investments, target portfolio allocations		50.00%
Corporate debt securities | Maximum
Schedule of Available-for-sale Securities [Line Items]
Investments, target portfolio allocations		15.00%
Other aggregated investments | Minimum | Securities of any one issuer (excluding the U.S. Government)
Schedule of Available-for-sale Securities [Line Items]
Investments, target portfolio allocations		10.00%
Other aggregated investments | Maximum | Securities of any one issuer (excluding the U.S. Government)
Schedule of Available-for-sale Securities [Line Items]
Investments, target portfolio allocations		15.00%
Investor | Fairholme Capital Management, L.L.C.
Schedule of Available-for-sale Securities [Line Items]
Common stock ownership percentage		32.50%