Debt and Equity Securities (Composition of Debt Securities Portfolio in Unrealized Loss Position by Credit Rating Agencies) (Detail) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value of debt securities, Unrealized loss position	$ 3,798,483	$ 2,643,947
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	$ 3,287,770
Percentage of debt securities in unrealized loss position	86.60%
Investment Grade | BBB+ to BBB-
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value of debt securities, Unrealized loss position	$ 355,577
Percentage of debt securities in unrealized loss position	9.40%
Non-Investment Grade
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value of debt securities, Unrealized loss position	$ 155,136
Percentage of debt securities in unrealized loss position	4.00%
U.S. Treasury Bonds
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value of debt securities, Unrealized loss position	$ 145,584	116,103
Percentage of debt securities in unrealized loss position	100.00%
U.S. Treasury Bonds | Investment Grade | A- or Higher
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value of debt securities, Unrealized loss position	$ 145,584
Percentage of debt securities in unrealized loss position	100.00%
U.S. Treasury Bonds | Investment Grade | 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%
U.S. Treasury Bonds | Non-Investment Grade
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%
Municipal Bonds
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value of debt securities, Unrealized loss position	$ 700,800	506,187
Percentage of debt securities in unrealized loss position	100.00%
Municipal Bonds | Investment Grade | A- or Higher
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value of debt securities, Unrealized loss position	$ 654,260
Percentage of debt securities in unrealized loss position	93.40%
Municipal Bonds | Investment Grade | BBB+ to BBB-
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value of debt securities, Unrealized loss position	$ 36,534
Percentage of debt securities in unrealized loss position	5.20%
Municipal Bonds | Non-Investment Grade
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value of debt securities, Unrealized loss position	$ 10,006
Percentage of debt securities in unrealized loss position	1.40%
Foreign Government Bonds
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value of debt securities, Unrealized loss position	$ 107,777	105,620
Percentage of debt securities in unrealized loss position	100.00%
Foreign Government Bonds | Investment Grade | A- or Higher
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value of debt securities, Unrealized loss position	$ 79,130
Percentage of debt securities in unrealized loss position	73.40%
Foreign Government Bonds | Investment Grade | BBB+ to BBB-
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value of debt securities, Unrealized loss position	$ 23,567
Percentage of debt securities in unrealized loss position	21.90%
Foreign Government Bonds | Non-Investment Grade
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value of debt securities, Unrealized loss position	$ 5,080
Percentage of debt securities in unrealized loss position	4.70%
Governmental Agency Bonds
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value of debt securities, Unrealized loss position	$ 234,666	149,444
Percentage of debt securities in unrealized loss position	100.00%
Governmental Agency Bonds | Investment Grade | A- or Higher
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value of debt securities, Unrealized loss position	$ 234,666
Percentage of debt securities in unrealized loss position	100.00%
Governmental Agency Bonds | Investment Grade | 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%
Governmental Agency Bonds | Non-Investment Grade
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%
Governmental Agency Mortgage-Backed Securities
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value of debt securities, Unrealized loss position	$ 1,836,133	1,446,830
Percentage of debt securities in unrealized loss position	100.00%
Governmental Agency Mortgage-Backed Securities | Investment Grade | A- or Higher
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value of debt securities, Unrealized loss position	$ 1,836,133
Percentage of debt securities in unrealized loss position	100.00%
Governmental Agency Mortgage-Backed Securities | Investment Grade | 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%
Governmental Agency Mortgage-Backed Securities | Non-Investment Grade
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%
U.S. Corporate Debt Securities
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value of debt securities, Unrealized loss position	$ 572,484	232,541
Percentage of debt securities in unrealized loss position	100.00%
U.S. Corporate Debt Securities | Investment Grade | A- or Higher
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value of debt securities, Unrealized loss position	$ 255,973
Percentage of debt securities in unrealized loss position	44.70%
U.S. Corporate Debt Securities | Investment Grade | BBB+ to BBB-
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value of debt securities, Unrealized loss position	$ 200,971
Percentage of debt securities in unrealized loss position	35.10%
U.S. Corporate Debt Securities | Non-Investment Grade
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value of debt securities, Unrealized loss position	$ 115,540
Percentage of debt securities in unrealized loss position	20.20%
Foreign Corporate Debt Securities
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value of debt securities, Unrealized loss position	$ 201,039	$ 87,222
Percentage of debt securities in unrealized loss position	100.00%
Foreign Corporate Debt Securities | Investment Grade | A- or Higher
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value of debt securities, Unrealized loss position	$ 82,024
Percentage of debt securities in unrealized loss position	40.80%
Foreign Corporate Debt Securities | Investment Grade | BBB+ to BBB-
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value of debt securities, Unrealized loss position	$ 94,505
Percentage of debt securities in unrealized loss position	47.00%
Foreign Corporate Debt Securities | Non-Investment Grade
Schedule Of Available For Sale Securities [Line Items]
Estimated fair value of debt securities, Unrealized loss position	$ 24,510
Percentage of debt securities in unrealized loss position	12.20%