Schedule I - Summary Of Investments-Other Than Investments In Related Parties (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014
Summary Of Investments Other Than Investments In Related Parties Reportable Data [Line Items]
Cost	$ 4,007,608
Market value	4,031,787
Amount at which shown in the balance sheet	4,033,892
Deposits with Banks
Summary Of Investments Other Than Investments In Related Parties Reportable Data [Line Items]
Cost	21,445
Market value	21,540
Amount at which shown in the balance sheet	21,445
U.S. Treasury Bonds
Summary Of Investments Other Than Investments In Related Parties Reportable Data [Line Items]
Cost	64,195
Market value	64,982
Amount at which shown in the balance sheet	64,982
Municipal Bonds
Summary Of Investments Other Than Investments In Related Parties Reportable Data [Line Items]
Cost	577,703
Market value	587,677
Amount at which shown in the balance sheet	587,677
Foreign Bonds
Summary Of Investments Other Than Investments In Related Parties Reportable Data [Line Items]
Cost	194,749
Market value	196,750
Amount at which shown in the balance sheet	196,750
Governmental Agency Bonds
Summary Of Investments Other Than Investments In Related Parties Reportable Data [Line Items]
Cost	198,330
Market value	197,874
Amount at which shown in the balance sheet	197,874
Governmental Agency Mortgage-Backed Securities
Summary Of Investments Other Than Investments In Related Parties Reportable Data [Line Items]
Cost	1,812,766
Market value	1,812,162
Amount at which shown in the balance sheet	1,812,162
Non-Agency Mortgage-Backed Securities
Summary Of Investments Other Than Investments In Related Parties Reportable Data [Line Items]
Cost	15,949
Market value	16,538
Amount at which shown in the balance sheet	16,538
Corporate Debt Securities
Summary Of Investments Other Than Investments In Related Parties Reportable Data [Line Items]
Cost	568,774
Market value	574,269
Amount at which shown in the balance sheet	574,269
Total Debt Securities
Summary Of Investments Other Than Investments In Related Parties Reportable Data [Line Items]
Cost	3,432,466
Market value	3,450,252
Amount at which shown in the balance sheet	3,450,252
Equity Securities
Summary Of Investments Other Than Investments In Related Parties Reportable Data [Line Items]
Cost	393,914
Market value	402,412
Amount at which shown in the balance sheet	402,412
Notes Receivable, Net
Summary Of Investments Other Than Investments In Related Parties Reportable Data [Line Items]
Cost	6,130
Market value	3,930
Amount at which shown in the balance sheet	6,130
Other Long-term Investments
Summary Of Investments Other Than Investments In Related Parties Reportable Data [Line Items]
Cost	153,653
Market value	153,653	[1]
Amount at which shown in the balance sheet	$ 153,653

[1] As other long-term investments are not publicly traded, reasonable estimate of the fair values could not be made without incurring excessive costs.