Debt and Equity Securities (Composition of Debt Securities Portfolio in Unrealized Loss Position by Credit Rating Agencies) (Detail) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value of debt securities, Unrealized loss position	$ 2,835,763	$ 3,373,514
Percentage of debt securities in unrealized loss position	100.00%
U.S. Treasury Bonds
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value of debt securities, Unrealized loss position	$ 42,098	75,364
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	$ 266,718	541,526
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	$ 49,378	65,773
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	$ 144,348	146,901
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	$ 1,905,041	1,833,069
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	$ 296,164	484,317
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	$ 132,016	$ 226,564
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	$ 2,538,170
Percentage of debt securities in unrealized loss position	89.50%
Investment Grade | BBB+ to BBB-
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value of debt securities, Unrealized loss position	$ 155,274
Percentage of debt securities in unrealized loss position	5.50%
Investment Grade | U.S. Treasury Bonds | A- or Higher
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value of debt securities, Unrealized loss position	$ 42,098
Percentage of debt securities in unrealized loss position	100.00%
Investment Grade | U.S. Treasury 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 | Municipal Bonds | A- or Higher
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value of debt securities, Unrealized loss position	$ 261,506
Percentage of debt securities in unrealized loss position	98.10%
Investment Grade | Municipal Bonds | BBB+ to BBB-
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value of debt securities, Unrealized loss position	$ 2,970
Percentage of debt securities in unrealized loss position	1.10%
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	$ 33,358
Percentage of debt securities in unrealized loss position	67.50%
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	$ 14,643
Percentage of debt securities in unrealized loss position	29.70%
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	$ 144,348
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	$ 1,905,041
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	$ 104,171
Percentage of debt securities in unrealized loss position	35.10%
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	$ 72,752
Percentage of debt securities in unrealized loss position	24.60%
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	$ 47,648
Percentage of debt securities in unrealized loss position	36.10%
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	$ 64,909
Percentage of debt securities in unrealized loss position	49.20%
Non-Investment Grade
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value of debt securities, Unrealized loss position	$ 142,319
Percentage of debt securities in unrealized loss position	5.00%
Non-Investment Grade | U.S. Treasury 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 | Municipal Bonds
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value of debt securities, Unrealized loss position	$ 2,242
Percentage of debt securities in unrealized loss position	0.80%
Non-Investment Grade | Foreign Government Bonds
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value of debt securities, Unrealized loss position	$ 1,377
Percentage of debt securities in unrealized loss position	2.80%
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	$ 119,241
Percentage of debt securities in unrealized loss position	40.30%
Non-Investment Grade | Foreign Corporate Debt Securities
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value of debt securities, Unrealized loss position	$ 19,459
Percentage of debt securities in unrealized loss position	14.70%