Debt and Equity Securities (Investments in Debt Securities, Classified as Available-For-Sale) (Detail) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Schedule Of Available For Sale Securities [Line Items]
Debt Securities, Amortized cost	$ 4,799,401	$ 4,603,021
Debt Securities, Gross unrealized gains	40,348	21,802
Debt Securities, Gross unrealized losses	(36,265)	(71,460)
Debt securities, Estimated fair value	4,803,484	4,553,363
U.S. Treasury Bonds
Schedule Of Available For Sale Securities [Line Items]
Debt Securities, Amortized cost	145,396	155,441
Debt Securities, Gross unrealized gains	1,484	416
Debt Securities, Gross unrealized losses	(1,427)	(4,466)
Debt securities, Estimated fair value	145,453	151,391
Municipal Bonds
Schedule Of Available For Sale Securities [Line Items]
Debt Securities, Amortized cost	1,068,611	1,004,659
Debt Securities, Gross unrealized gains	14,546	6,340
Debt Securities, Gross unrealized losses	(10,795)	(26,666)
Debt securities, Estimated fair value	1,072,362	984,333
Foreign Government Bonds
Schedule Of Available For Sale Securities [Line Items]
Debt Securities, Amortized cost	162,550	141,887
Debt Securities, Gross unrealized gains	556	600
Debt Securities, Gross unrealized losses	(1,457)	(2,439)
Debt securities, Estimated fair value	161,649	140,048
Governmental Agency Bonds
Schedule Of Available For Sale Securities [Line Items]
Debt Securities, Amortized cost	223,546	197,343
Debt Securities, Gross unrealized gains	1,063	691
Debt Securities, Gross unrealized losses	(2,891)	(4,166)
Debt securities, Estimated fair value	221,718	193,868
Governmental Agency Mortgage-Backed Securities
Schedule Of Available For Sale Securities [Line Items]
Debt Securities, Amortized cost	2,238,222	2,187,482
Debt Securities, Gross unrealized gains	4,098	2,983
Debt Securities, Gross unrealized losses	(16,400)	(26,792)
Debt securities, Estimated fair value	2,225,920	2,163,673
U.S. Corporate Debt Securities
Schedule Of Available For Sale Securities [Line Items]
Debt Securities, Amortized cost	716,299	675,683
Debt Securities, Gross unrealized gains	13,400	8,282
Debt Securities, Gross unrealized losses	(2,771)	(5,441)
Debt securities, Estimated fair value	726,928	678,524
Foreign Corporate Debt Securities
Schedule Of Available For Sale Securities [Line Items]
Debt Securities, Amortized cost	244,777	240,526
Debt Securities, Gross unrealized gains	5,201	2,490
Debt Securities, Gross unrealized losses	(524)	(1,490)
Debt securities, Estimated fair value	$ 249,454	$ 241,526