Debt and Equity Securities (Narrative) (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Schedule Of Debt and Equity Securities [Line Items]
Realized gains on sales of debt securities	$ 600	$ 900	$ 1,900	$ 3,900
Realized losses on sales of debt securities	3,100	400	6,900	4,000
Proceeds from sale of debt securities	183,100	63,700	525,400	357,700
Realized and unrealized net gains on sales of equity securities	14,200	700	16,000	17,900
Net unrealized gains on equity securities	14,100		15,900
Realized losses on sales of equity securities		$ 300		$ 2,000
Debt securities, Estimated fair value	5,529,860		5,529,860		$ 4,752,684
Estimated fair value of debt securities, Unrealized loss position	4,412,496		4,412,496		$ 2,643,947
Bank Loans
Schedule Of Debt and Equity Securities [Line Items]
Debt securities, Estimated fair value	157,100		157,100
Estimated fair value of debt securities, Unrealized loss position	28,900		28,900
Emerging Market Securities
Schedule Of Debt and Equity Securities [Line Items]
Debt securities, Estimated fair value	86,300		86,300
Estimated fair value of debt securities, Unrealized loss position	72,900		72,900
Non-Investment Grade
Schedule Of Debt and Equity Securities [Line Items]
Debt securities, Estimated fair value	277,916		277,916
Estimated fair value of debt securities, Unrealized loss position	112,832		112,832
Non-Investment Grade | Bank Loans
Schedule Of Debt and Equity Securities [Line Items]
Debt securities, Estimated fair value	146,300		146,300
Estimated fair value of debt securities, Unrealized loss position	28,500		28,500
Non-Investment Grade | High Yield Corporate Debt Securities
Schedule Of Debt and Equity Securities [Line Items]
Debt securities, Estimated fair value	95,600		95,600
Estimated fair value of debt securities, Unrealized loss position	60,600		60,600
Non-Investment Grade | Emerging Market Securities
Schedule Of Debt and Equity Securities [Line Items]
Debt securities, Estimated fair value	11,600		11,600
Estimated fair value of debt securities, Unrealized loss position	$ 10,100		$ 10,100