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Commitments and Contingencies - Additional Information (Detail) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended
	Jun. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017
Commitments and Contingencies Disclosure [Line Items]
Rent expense	$ 9,700	$ 21,800
Investments, debt securities	14,129,577	14,129,577		$ 11,823,968
Investments, equity securities	2,292,803	2,292,803		3,572,790
Stockholders' equity	8,685,914	$ 8,685,914	$ 8,236,487	$ 7,692,710	$ 8,404,389	$ 8,373,400	$ 8,514,063
Maximum
Commitments and Contingencies Disclosure [Line Items]
Non-cancelable operating leases, expiration date		2038
Energy Sector Business
Commitments and Contingencies Disclosure [Line Items]
Investments and equity in consolidated subsidiaries	416,100	$ 416,100
Investments, debt securities	323,900	323,900
Investments, equity securities	5,400	5,400
Energy Sector Business \| Stranded Oil Resources Corporation
Commitments and Contingencies Disclosure [Line Items]
Stockholders' equity	$ 86,800	$ 86,800