Debt and Equity Securities (Investments in Debt Securities, Classified as Available-For-Sale) (Detail) - USD ($)
$ in Thousands |
Mar. 31, 2016 |
Dec. 31, 2015 |
| Schedule Of Available For Sale Securities [Line Items] |
|
|
| Debt Securities, Amortized Cost |
$ 4,224,758
|
$ 4,290,864
|
| Debt Securities, Gross unrealized gains |
50,726
|
25,745
|
| Debt Securities, Gross unrealized losses |
(16,539)
|
(37,262)
|
| Debt securities, Estimated fair value |
4,258,945
|
4,279,347
|
| U.S. Treasury Bonds |
|
|
| Schedule Of Available For Sale Securities [Line Items] |
|
|
| Debt Securities, Amortized Cost |
142,415
|
130,252
|
| Debt Securities, Gross unrealized gains |
1,998
|
421
|
| Debt Securities, Gross unrealized losses |
(19)
|
(1,301)
|
| Debt securities, Estimated fair value |
144,394
|
129,372
|
| Municipal Bonds |
|
|
| Schedule Of Available For Sale Securities [Line Items] |
|
|
| Debt Securities, Amortized Cost |
682,795
|
692,000
|
| Debt Securities, Gross unrealized gains |
17,384
|
12,640
|
| Debt Securities, Gross unrealized losses |
(655)
|
(845)
|
| Debt securities, Estimated fair value |
699,524
|
703,795
|
| Foreign Government Bonds |
|
|
| Schedule Of Available For Sale Securities [Line Items] |
|
|
| Debt Securities, Amortized Cost |
128,507
|
129,984
|
| Debt Securities, Gross unrealized gains |
1,378
|
1,132
|
| Debt Securities, Gross unrealized losses |
(308)
|
(1,015)
|
| Debt securities, Estimated fair value |
129,577
|
130,101
|
| Governmental Agency Bonds |
|
|
| Schedule Of Available For Sale Securities [Line Items] |
|
|
| Debt Securities, Amortized Cost |
353,205
|
419,869
|
| Debt Securities, Gross unrealized gains |
2,161
|
1,023
|
| Debt Securities, Gross unrealized losses |
(425)
|
(2,801)
|
| Debt securities, Estimated fair value |
354,941
|
418,091
|
| Governmental Agency Mortgage-Backed Securities |
|
|
| Schedule Of Available For Sale Securities [Line Items] |
|
|
| Debt Securities, Amortized Cost |
2,039,111
|
2,065,728
|
| Debt Securities, Gross unrealized gains |
11,893
|
4,984
|
| Debt Securities, Gross unrealized losses |
(6,046)
|
(15,039)
|
| Debt securities, Estimated fair value |
2,044,958
|
2,055,673
|
| U.S. Corporate Debt Securities |
|
|
| Schedule Of Available For Sale Securities [Line Items] |
|
|
| Debt Securities, Amortized Cost |
653,692
|
642,869
|
| Debt Securities, Gross unrealized gains |
12,868
|
4,297
|
| Debt Securities, Gross unrealized losses |
(6,695)
|
(12,483)
|
| Debt securities, Estimated fair value |
659,865
|
634,683
|
| Foreign Corporate Debt Securities |
|
|
| Schedule Of Available For Sale Securities [Line Items] |
|
|
| Debt Securities, Amortized Cost |
225,033
|
210,162
|
| Debt Securities, Gross unrealized gains |
3,044
|
1,248
|
| Debt Securities, Gross unrealized losses |
(2,391)
|
(3,778)
|
| Debt securities, Estimated fair value |
$ 225,686
|
$ 207,632
|