Debt and Equity Securities (Investments in Debt Securities, Classified as Available-For-Sale) (Detail) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Schedule Of Available For Sale Securities [Line Items]
Debt Securities, Amortized cost	$ 5,928,878	$ 5,761,150
Debt Securities, Gross unrealized gains	62,735	23,361
Debt Securities, Gross unrealized losses	(32,268)	(70,700)
Debt securities, Estimated fair value	5,959,345	5,713,811
U.S. Treasury Bonds
Schedule Of Available For Sale Securities [Line Items]
Debt Securities, Amortized cost	148,860	162,904
Debt Securities, Gross unrealized gains	1,278	741
Debt Securities, Gross unrealized losses	(324)	(1,139)
Debt securities, Estimated fair value	149,814	162,506
Municipal Bonds
Schedule Of Available For Sale Securities [Line Items]
Debt Securities, Amortized cost	1,058,106	1,050,134
Debt Securities, Gross unrealized gains	22,510	7,210
Debt Securities, Gross unrealized losses	(2,197)	(12,309)
Debt securities, Estimated fair value	1,078,419	1,045,035
Foreign Government Bonds
Schedule Of Available For Sale Securities [Line Items]
Debt Securities, Amortized cost	152,539	158,885
Debt Securities, Gross unrealized gains	1,285	571
Debt Securities, Gross unrealized losses	(472)	(2,159)
Debt securities, Estimated fair value	153,352	157,297
Governmental Agency Bonds
Schedule Of Available For Sale Securities [Line Items]
Debt Securities, Amortized cost	352,350	319,115
Debt Securities, Gross unrealized gains	2,577	1,145
Debt Securities, Gross unrealized losses	(2,372)	(4,093)
Debt securities, Estimated fair value	352,555	316,167
Governmental Agency Mortgage-Backed Securities
Schedule Of Available For Sale Securities [Line Items]
Debt Securities, Amortized cost	3,331,671	3,219,585
Debt Securities, Gross unrealized gains	24,421	12,030
Debt Securities, Gross unrealized losses	(20,810)	(29,016)
Debt securities, Estimated fair value	3,335,282	3,202,599
U.S. Corporate Debt Securities
Schedule Of Available For Sale Securities [Line Items]
Debt Securities, Amortized cost	596,614	575,646
Debt Securities, Gross unrealized gains	7,677	1,113
Debt Securities, Gross unrealized losses	(4,456)	(15,499)
Debt securities, Estimated fair value	599,835	561,260
Foreign Corporate Debt Securities
Schedule Of Available For Sale Securities [Line Items]
Debt Securities, Amortized cost	288,738	274,881
Debt Securities, Gross unrealized gains	2,987	551
Debt Securities, Gross unrealized losses	(1,637)	(6,485)
Debt securities, Estimated fair value	$ 290,088	$ 268,947

Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27161-111563

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Amount, before tax, of unrealized loss in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27161-111563

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Amortized cost of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27161-111563

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Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=109979856&loc=d3e26610-111562

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