Debt and Equity Securities (Investments in Debt Securities, Classified as Available-For-Sale) (Detail) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016
Schedule Of Available For Sale Securities [Line Items]
Debt Securities, Amortized cost	$ 4,611,976	$ 4,603,021
Debt Securities, Gross unrealized gains	27,273	21,802
Debt Securities, Gross unrealized losses	(55,157)	(71,460)
Debt securities, Estimated fair value	4,584,092	4,553,363
U.S. Treasury Bonds
Schedule Of Available For Sale Securities [Line Items]
Debt Securities, Amortized cost	166,012	155,441
Debt Securities, Gross unrealized gains	589	416
Debt Securities, Gross unrealized losses	(1,999)	(4,466)
Debt securities, Estimated fair value	164,602	151,391
Municipal Bonds
Schedule Of Available For Sale Securities [Line Items]
Debt Securities, Amortized cost	1,022,337	1,004,659
Debt Securities, Gross unrealized gains	8,817	6,340
Debt Securities, Gross unrealized losses	(21,068)	(26,666)
Debt securities, Estimated fair value	1,010,086	984,333
Foreign Government Bonds
Schedule Of Available For Sale Securities [Line Items]
Debt Securities, Amortized cost	136,763	141,887
Debt Securities, Gross unrealized gains	690	600
Debt Securities, Gross unrealized losses	(1,031)	(2,439)
Debt securities, Estimated fair value	136,422	140,048
Governmental Agency Bonds
Schedule Of Available For Sale Securities [Line Items]
Debt Securities, Amortized cost	199,379	197,343
Debt Securities, Gross unrealized gains	799	691
Debt Securities, Gross unrealized losses	(3,852)	(4,166)
Debt securities, Estimated fair value	196,326	193,868
Governmental Agency Mortgage-Backed Securities
Schedule Of Available For Sale Securities [Line Items]
Debt Securities, Amortized cost	2,152,814	2,187,482
Debt Securities, Gross unrealized gains	3,543	2,983
Debt Securities, Gross unrealized losses	(22,028)	(26,792)
Debt securities, Estimated fair value	2,134,329	2,163,673
U.S. Corporate Debt Securities
Schedule Of Available For Sale Securities [Line Items]
Debt Securities, Amortized cost	694,145	675,683
Debt Securities, Gross unrealized gains	9,205	8,282
Debt Securities, Gross unrealized losses	(4,514)	(5,441)
Debt securities, Estimated fair value	698,836	678,524
Foreign Corporate Debt Securities
Schedule Of Available For Sale Securities [Line Items]
Debt Securities, Amortized cost	240,526	240,526
Debt Securities, Gross unrealized gains	3,630	2,490
Debt Securities, Gross unrealized losses	(665)	(1,490)
Debt securities, Estimated fair value	$ 243,491	$ 241,526

Amount before tax of unrealized gain in accumulated other comprehensive income (AOCI) on investments in debt securities classified as available-for-sale.

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Amount before tax of unrealized loss in accumulated other comprehensive income (AOCI) on investments in debt securities classified as available-for-sale.

+ References

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This item represents the cost of debt securities, which are categorized neither as held-to-maturity nor trading, net of adjustments including accretion, amortization, collection of cash, previous other-than-temporary impairments recognized in earnings (less any cumulative-effect adjustments recognized, as defined), and fair value hedge accounting adjustments, if any.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (aa)
-URI http://asc.fasb.org/extlink&oid=73527792&loc=d3e27161-111563

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.2)
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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Amount of debt securities categorized neither as held-to-maturity nor trading.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 25
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=65884222&loc=d3e22054-111558

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=66010992&loc=d3e26610-111562

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (aa)
-URI http://asc.fasb.org/extlink&oid=73527792&loc=d3e27161-111563

Reference 4: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 320
-Section 50
-Paragraph 2
-Subparagraph (b)-(g)
-URI http://asc.fasb.org/extlink&oid=6957658&loc=d3e62557-112803

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