Investment in Securities - Bond by Credit Quality Rating Distribution (Detail) - Bonds - Credit Risk Financial Instruments - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Concentration Risk [Line Items]
Amortized cost	$ 15,066,931	$ 14,473,070
Estimated fair value	$ 15,693,775	$ 14,345,647
Percent of fair value	100.00%	100.00%
AAA
Concentration Risk [Line Items]
Amortized cost	$ 722,542	$ 690,009
Estimated fair value	$ 751,560	$ 702,531
Percent of fair value	4.80%	4.90%
AA
Concentration Risk [Line Items]
Amortized cost	$ 1,174,698	$ 1,326,947
Estimated fair value	$ 1,224,210	$ 1,336,380
Percent of fair value	7.80%	9.30%
A
Concentration Risk [Line Items]
Amortized cost	$ 5,589,846	$ 5,350,316
Estimated fair value	$ 5,835,821	$ 5,314,589
Percent of fair value	37.10%	37.00%
BBB
Concentration Risk [Line Items]
Amortized cost	$ 7,045,065	$ 6,584,478
Estimated fair value	$ 7,352,915	$ 6,507,212
Percent of fair value	46.90%	45.40%
BB and Below
Concentration Risk [Line Items]
Amortized cost	$ 534,780	$ 521,320
Estimated fair value	$ 529,269	$ 484,935
Percent of fair value	3.40%	3.40%