Debt and Equity Securities (Narrative) (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Schedule Of Debt and Equity Securities [Line Items]
Realized gains on sales of debt securities	$ 700	$ 1,300	$ 1,300	$ 2,900
Realized losses on sales of debt securities	2,600	300	3,800	3,500
Proceeds from sale of debt securities	186,600	139,600	342,300	294,000
Realized net gains on sales of equity securities	7,700	15,400	1,800	17,200
Net unrealized gains on equity securities	5,600		1,800
Realized losses on sales of equity securities		$ 1,600		$ 1,700
Debt securities, Estimated fair value	5,119,843		5,119,843		$ 4,752,684
Estimated fair value of debt securities, Unrealized loss position	3,798,483		3,798,483		$ 2,643,947
Bank Loans
Schedule Of Debt and Equity Securities [Line Items]
Debt securities, Estimated fair value	155,400		155,400
Estimated fair value of debt securities, Unrealized loss position	74,400		74,400
Emerging Market Securities
Schedule Of Debt and Equity Securities [Line Items]
Debt securities, Estimated fair value	84,400		84,400
Estimated fair value of debt securities, Unrealized loss position	72,700		72,700
Non-Investment Grade
Schedule Of Debt and Equity Securities [Line Items]
Debt securities, Estimated fair value	260,895		260,895
Estimated fair value of debt securities, Unrealized loss position	155,136		155,136
Non-Investment Grade | Bank Loans
Schedule Of Debt and Equity Securities [Line Items]
Debt securities, Estimated fair value	141,900		141,900
Estimated fair value of debt securities, Unrealized loss position	70,200		70,200
Non-Investment Grade | High Yield Corporate Debt Securities
Schedule Of Debt and Equity Securities [Line Items]
Debt securities, Estimated fair value	84,500		84,500
Estimated fair value of debt securities, Unrealized loss position	64,700		64,700
Non-Investment Grade | Emerging Market Securities
Schedule Of Debt and Equity Securities [Line Items]
Debt securities, Estimated fair value	11,200		11,200
Estimated fair value of debt securities, Unrealized loss position	$ 10,200		$ 10,200