Investment in Securities - Bond by Credit Quality Rating Distribution (Detail) - Bonds - Credit Risk Financial Instruments - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Concentration Risk [Line Items]
Amortized cost	$ 14,473,070	$ 13,510,860
Estimated fair value	$ 14,345,647	$ 13,919,661
Percent of fair value	100.00%	100.00%
AAA
Concentration Risk [Line Items]
Amortized cost	$ 690,009	$ 638,039
Estimated fair value	$ 702,531	$ 664,396
Percent of fair value	4.90%	4.80%
AA
Concentration Risk [Line Items]
Amortized cost	$ 1,326,947	$ 1,220,544
Estimated fair value	$ 1,336,380	$ 1,264,282
Percent of fair value	9.30%	9.00%
S&P A Rating
Concentration Risk [Line Items]
Amortized cost	$ 5,350,316	$ 4,856,802
Estimated fair value	$ 5,314,589	$ 4,997,574
Percent of fair value	37.00%	35.90%
S&P BBB Rating
Concentration Risk [Line Items]
Amortized cost	$ 6,584,478	$ 6,273,220
Estimated fair value	$ 6,507,212	$ 6,480,719
Percent of fair value	45.40%	46.60%
BB and Below
Concentration Risk [Line Items]
Amortized cost	$ 521,320	$ 522,255
Estimated fair value	$ 484,935	$ 512,690
Percent of fair value	3.40%	3.70%