Debt and Equity Securities (Narrative) (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018
Schedule Of Debt and Equity Securities [Line Items]
Realized gains on sales of debt securities	$ 3,100	$ 700
Realized losses on sales of debt securities	3,000	1,200
Proceeds from sale of debt securities	391,400	155,700
Realized and unrealized net gains (losses) on equity securities	32,500	(5,900)
Net unrealized gains on equity securities	29,000
Net unrealized losses on equity securities		$ 6,500
Debt securities, Estimated fair value	5,959,345		$ 5,713,811
Estimated fair value of debt securities, Unrealized loss position	2,835,763		$ 3,373,514
Bank Loans
Schedule Of Debt and Equity Securities [Line Items]
Debt securities, Estimated fair value	148,900
Estimated fair value of debt securities, Unrealized loss position	134,900
Emerging Market Securities
Schedule Of Debt and Equity Securities [Line Items]
Debt securities, Estimated fair value	69,800
Estimated fair value of debt securities, Unrealized loss position	29,500
Non-Investment Grade
Schedule Of Debt and Equity Securities [Line Items]
Debt securities, Estimated fair value	191,146
Estimated fair value of debt securities, Unrealized loss position	142,319
Non-Investment Grade | Bank Loans
Schedule Of Debt and Equity Securities [Line Items]
Debt securities, Estimated fair value	136,400
Estimated fair value of debt securities, Unrealized loss position	124,800
Non-Investment Grade | High Yield Corporate Debt Securities
Schedule Of Debt and Equity Securities [Line Items]
Debt securities, Estimated fair value	38,700
Estimated fair value of debt securities, Unrealized loss position	11,200
Non-Investment Grade | Emerging Market Securities
Schedule Of Debt and Equity Securities [Line Items]
Debt securities, Estimated fair value	10,000
Estimated fair value of debt securities, Unrealized loss position	$ 4,100