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Commitments and Contingencies - Additional Information (Detail) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
	Sep. 30, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017
Commitments and Contingencies Disclosure [Line Items]
Rent expense	$ 10,400	$ 32,200
Investments, debt securities	14,488,289	14,488,289			$ 11,823,968
Investments, equity securities	2,092,267	2,092,267			3,572,790
Stockholders' equity	8,828,658	$ 8,828,658	$ 8,685,914	$ 8,236,487	$ 7,692,710	$ 8,595,085	$ 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	432,500	$ 432,500
Investments, debt securities	340,900	340,900
Investments, equity securities	3,800	3,800
Energy Sector Business \| Stranded Oil Resources Corporation
Commitments and Contingencies Disclosure [Line Items]
Stockholders' equity	$ 87,800	$ 87,800