Debt and Equity Securities (Composition of Debt Securities Portfolio in Unrealized Loss Position by Credit Rating Agencies) (Detail) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value of debt securities, Unrealized loss position	$ 1,454,543	$ 1,663,146
Percentage of debt securities in unrealized loss position	100.00%
Municipal bonds
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value of debt securities, Unrealized loss position	$ 65,600	32,160
Percentage of debt securities in unrealized loss position	100.00%
Foreign government bonds
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value of debt securities, Unrealized loss position	$ 32,427	123,168
Percentage of debt securities in unrealized loss position	100.00%
Governmental agency bonds
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value of debt securities, Unrealized loss position	$ 1,540	39,416
Percentage of debt securities in unrealized loss position	100.00%
Governmental agency mortgage-backed securities
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value of debt securities, Unrealized loss position	$ 930,455	1,356,611
Percentage of debt securities in unrealized loss position	100.00%
U.S. corporate debt securities
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value of debt securities, Unrealized loss position	$ 228,416	52,823
Percentage of debt securities in unrealized loss position	100.00%
Foreign corporate debt securities
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value of debt securities, Unrealized loss position	$ 196,105	$ 43,268
Percentage of debt securities in unrealized loss position	100.00%
Investment Grade | A- or Higher
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value of debt securities, Unrealized loss position	$ 1,139,302
Percentage of debt securities in unrealized loss position	78.40%
Investment Grade | BBB+ to BBB-
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value of debt securities, Unrealized loss position	$ 236,188
Percentage of debt securities in unrealized loss position	16.20%
Investment Grade | Municipal bonds | A- or Higher
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value of debt securities, Unrealized loss position	$ 46,321
Percentage of debt securities in unrealized loss position	70.60%
Investment Grade | Municipal bonds | BBB+ to BBB-
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value of debt securities, Unrealized loss position	$ 19,279
Percentage of debt securities in unrealized loss position	29.40%
Investment Grade | Foreign government bonds | A- or Higher
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value of debt securities, Unrealized loss position	$ 24,960
Percentage of debt securities in unrealized loss position	77.00%
Investment Grade | Foreign government bonds | BBB+ to BBB-
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value of debt securities, Unrealized loss position	$ 6,139
Percentage of debt securities in unrealized loss position	18.90%
Investment Grade | Governmental agency bonds | A- or Higher
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value of debt securities, Unrealized loss position	$ 1,540
Percentage of debt securities in unrealized loss position	100.00%
Investment Grade | Governmental agency bonds | BBB+ to BBB-
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value of debt securities, Unrealized loss position	$ 0
Percentage of debt securities in unrealized loss position	0.00%
Investment Grade | Governmental agency mortgage-backed securities | A- or Higher
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value of debt securities, Unrealized loss position	$ 930,455
Percentage of debt securities in unrealized loss position	100.00%
Investment Grade | Governmental agency mortgage-backed securities | BBB+ to BBB-
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value of debt securities, Unrealized loss position	$ 0
Percentage of debt securities in unrealized loss position	0.00%
Investment Grade | U.S. corporate debt securities | A- or Higher
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value of debt securities, Unrealized loss position	$ 68,401
Percentage of debt securities in unrealized loss position	30.00%
Investment Grade | U.S. corporate debt securities | BBB+ to BBB-
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value of debt securities, Unrealized loss position	$ 102,437
Percentage of debt securities in unrealized loss position	44.80%
Investment Grade | Foreign corporate debt securities | A- or Higher
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value of debt securities, Unrealized loss position	$ 67,625
Percentage of debt securities in unrealized loss position	34.50%
Investment Grade | Foreign corporate debt securities | BBB+ to BBB-
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value of debt securities, Unrealized loss position	$ 108,333
Percentage of debt securities in unrealized loss position	55.20%
Non-Investment Grade
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value of debt securities, Unrealized loss position	$ 79,063
Percentage of debt securities in unrealized loss position	5.40%
Non-Investment Grade | Foreign government bonds
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value of debt securities, Unrealized loss position	$ 1,328
Percentage of debt securities in unrealized loss position	4.10%
Non-Investment Grade | Governmental agency bonds
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value of debt securities, Unrealized loss position	$ 0
Percentage of debt securities in unrealized loss position	0.00%
Non-Investment Grade | Governmental agency mortgage-backed securities
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value of debt securities, Unrealized loss position	$ 0
Percentage of debt securities in unrealized loss position	0.00%
Non-Investment Grade | U.S. corporate debt securities
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value of debt securities, Unrealized loss position	$ 57,578
Percentage of debt securities in unrealized loss position	25.20%
Non-Investment Grade | Foreign corporate debt securities
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value of debt securities, Unrealized loss position	$ 20,147
Percentage of debt securities in unrealized loss position	10.30%