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Debt and Equity Securities (Narrative) (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016	Dec. 31, 2016
Schedule Of Available For Sale Securities [Line Items]
Realized gains on sales of securities	$ 3,400	$ 1,000
Realized losses on sales of securities	3,300	$ 2,400
Debt securities, Estimated fair value	4,584,092		$ 4,553,363
Estimated fair value, Unrealized loss position	2,689,425		3,137,507
Debt Securities
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Unrealized loss position	2,597,614		$ 3,008,227
Bank Loans
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	151,700
Bank Loans \| Debt Securities
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Unrealized loss position	22,400
Emerging Market Securities
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	93,800
Emerging Market Securities \| Debt Securities
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Unrealized loss position	29,900
Non-Investment Grade
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	277,294
Non-Investment Grade \| Debt Securities
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Unrealized loss position	61,195
Non-Investment Grade \| Bank Loans
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	142,700
Non-Investment Grade \| Bank Loans \| Debt Securities
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Unrealized loss position	22,100
Non-Investment Grade \| High Yield Corporate Debt Securities
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	115,200
Non-Investment Grade \| High Yield Corporate Debt Securities \| Debt Securities
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Unrealized loss position	29,700
Non-Investment Grade \| Emerging Market Securities
Schedule Of Available For Sale Securities [Line Items]
Debt securities, Estimated fair value	14,400
Non-Investment Grade \| Emerging Market Securities \| Debt Securities
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value, Unrealized loss position	$ 4,400