Debt and Equity Securities (Narrative) (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Dec. 31, 2019
Schedule Of Debt and Equity Securities [Line Items]
Realized gains on sales of debt securities	$ 6,200	$ 3,100
Realized losses on sales of debt securities	1,300	3,000
Proceeds from sale of debt securities	209,400	391,400
Accrued interest receivable	27,200
Realized and unrealized net gains (losses) on equity securities	(82,500)	32,500
Net unrealized gains (losses) on equity securities	(82,400)	$ 29,000
Debt securities, Estimated fair value	5,839,530		$ 5,913,636
Estimated fair value of debt securities, Unrealized loss position	1,454,543		$ 1,663,146
Bank Loans
Schedule Of Debt and Equity Securities [Line Items]
Debt securities, Estimated fair value	67,700
Estimated fair value of debt securities, Unrealized loss position	60,900
Emerging Market Securities
Schedule Of Debt and Equity Securities [Line Items]
Debt securities, Estimated fair value	62,100
Estimated fair value of debt securities, Unrealized loss position	35,900
Non-Investment Grade
Schedule Of Debt and Equity Securities [Line Items]
Debt securities, Estimated fair value	100,072
Estimated fair value of debt securities, Unrealized loss position	79,063
Non-Investment Grade | Bank Loans
Schedule Of Debt and Equity Securities [Line Items]
Debt securities, Estimated fair value	61,400
Estimated fair value of debt securities, Unrealized loss position	54,700
Non-Investment Grade | High Yield Corporate Debt Securities
Schedule Of Debt and Equity Securities [Line Items]
Debt securities, Estimated fair value	30,600
Estimated fair value of debt securities, Unrealized loss position	20,100
Non-Investment Grade | Emerging Market Securities
Schedule Of Debt and Equity Securities [Line Items]
Debt securities, Estimated fair value	6,700
Estimated fair value of debt securities, Unrealized loss position	$ 4,300