Investment in Securities - Bond by Credit Quality Rating Distribution (Detail) - Bonds - Credit Risk Financial Instruments - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2019	Dec. 31, 2018
Concentration Risk [Line Items]
Amortized cost	$ 15,260,927	$ 14,473,070
Estimated fair value	$ 15,889,933	$ 14,345,647
Percent of fair value	100.00%	100.00%
AAA
Concentration Risk [Line Items]
Amortized cost	$ 764,050	$ 690,009
Estimated fair value	$ 795,776	$ 702,531
Percent of fair value	5.00%	4.90%
AA
Concentration Risk [Line Items]
Amortized cost	$ 1,219,171	$ 1,326,947
Estimated fair value	$ 1,274,409	$ 1,336,380
Percent of fair value	8.00%	9.30%
A
Concentration Risk [Line Items]
Amortized cost	$ 5,702,178	$ 5,350,316
Estimated fair value	$ 5,959,712	$ 5,314,589
Percent of fair value	37.50%	37.00%
BBB
Concentration Risk [Line Items]
Amortized cost	$ 7,101,873	$ 6,584,478
Estimated fair value	$ 7,399,545	$ 6,507,212
Percent of fair value	46.60%	45.40%
BB and below
Concentration Risk [Line Items]
Amortized cost	$ 473,655	$ 521,320
Estimated fair value	$ 460,491	$ 484,935
Percent of fair value	2.90%	3.40%