Investments - Narrative (Details)		3 Months Ended
		Mar. 31, 2018 USD ($) member	Mar. 31, 2017 USD ($)	Dec. 31, 2017 USD ($)
Schedule of Available-for-sale Securities [Line Items]
Gross realized loss		$ (1,100,000)
Sales of investments - debt securities		31,958,000	$ 48,729,000
Gross realized gain		(1,078,000)	3,122,000
Sales of available-for-sale securities			57,100,000
Unrealized losses, debt securities		1,300,000
Unrealized losses, all investments				$ 3,207,000
Other-than-temporary impairment loss		63,000	366,000
Investments - equity securities		44,928,000
Unrealized loss on investments, net		538,000	0
Reclassification into retained earnings	[1]	$ 932,000	$ 0
Number of investment committee members required to authorize investment | member		2
U.S. Treasury securities
Schedule of Available-for-sale Securities [Line Items]
Unrealized losses, debt securities		$ 25,000
Unrealized losses, all investments				22,000
Corporate debt securities
Schedule of Available-for-sale Securities [Line Items]
Unrealized losses, debt securities		$ 1,245,000
Unrealized losses, all investments				$ 1,817,000
Required percent of investment account held in cash or cash equivalents | Minimum
Schedule of Available-for-sale Securities [Line Items]
Investments, target portfolio allocations		25.00%
Other aggregated investments | Minimum | Securities of any one issuer (excluding the U.S. Government)
Schedule of Available-for-sale Securities [Line Items]
Investments, portfolio allocations requiring additional consent		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%
Investments, portfolio allocations requiring additional consent		15.00%
Other aggregated investments | Maximum | Single Issuer, exchange-traded common equities
Schedule of Available-for-sale Securities [Line Items]
Investments, target portfolio allocations		5.00%
Common stock investments | Maximum
Schedule of Available-for-sale Securities [Line Items]
Investments, target portfolio allocations, amount		$ 100,000,000.0
Common, preferred or other equity investments | Maximum
Schedule of Available-for-sale Securities [Line Items]
Investments, target portfolio allocations		25.00%
Investor | Fairholme Capital Management, L.L.C. , Fairholme Trust Company, LLC, And Mr. Berkowitz
Schedule of Available-for-sale Securities [Line Items]
Common stock ownership percentage		42.19%

[1] (1) The reclassification into retained earnings relates to the adoption of Accounting Standards Update (“ASU”) 2016-01 Financial Instruments - Overall, as amended (“ASU 2016-01”). The new guidance is effective January 1, 2018, and requires equity investments to be measured at fair value with changes in fair value recognized in results of operations rather than the condensed consolidated statements of comprehensive income. See Note 2. Summary of Significant Accounting Policies.