Condensed Financial Information - Balance Sheets (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
ASSETS
Cash and investments	$ 2,820,486us-gaap_InvestmentsAndCash	$ 2,574,586us-gaap_InvestmentsAndCash
Goodwill and other intangible assets	7,013	7,177
Other assets	44,384us-gaap_OtherAssets	35,039us-gaap_OtherAssets
Total assets	4,464,176us-gaap_Assets	4,169,452us-gaap_Assets	4,007,670us-gaap_Assets
LIABILITIES AND STOCKHOLDERS' EQUITY
Senior Notes	263,440us-gaap_SeniorNotes	263,308us-gaap_SeniorNotes
Accounts payable and other liabilities	93,336	69,379
Total liabilities	3,436,952us-gaap_Liabilities	3,267,240us-gaap_Liabilities
Preferred stock, $.10 par value, authorized 1,000,000 shares, none issued
Common stock, $.10 par value, authorized 50,000,000 shares, issued 17,792,846 shares for 2014 and 17,709,876 shares for 2013	1,778us-gaap_CommonStockValue	1,770us-gaap_CommonStockValue
Additional paid-in capital	347,022	335,546
Treasury stock, at cost (3,511,380 shares for 2014 and 2013)	(155,801)us-gaap_TreasuryStockValue	(155,801)us-gaap_TreasuryStockValue
Retained earnings	787,666	692,337
Accumulated other comprehensive income:
Total stockholders' equity	1,027,224us-gaap_StockholdersEquity	902,212us-gaap_StockholdersEquity	879,485us-gaap_StockholdersEquity	803,435us-gaap_StockholdersEquity
Total liabilities and stockholders' equity	4,464,176	4,169,452
Parent Company
ASSETS
Cash and investments	101,032	100,676
Investments in subsidiaries	1,163,822	1,040,214
Goodwill and other intangible assets	2,534	2,534
Other assets	27,531	26,538
Total assets	1,294,919	1,169,962
LIABILITIES AND STOCKHOLDERS' EQUITY
Senior Notes	263,440	263,308
Accounts payable and other liabilities	1,081	802
Accrued interest payable	3,174	3,640
Total liabilities	267,695	267,750
Preferred stock, $.10 par value, authorized 1,000,000 shares, none issued
Common stock, $.10 par value, authorized 50,000,000 shares, issued 17,792,846 shares for 2014 and 17,709,876 shares for 2013	1,778	1,770
Additional paid-in capital	347,022	335,546
Treasury stock, at cost (3,511,380 shares for 2014 and 2013)	(155,801)	(155,801)
Retained earnings	787,666	692,337
Accumulated other comprehensive income:
Net unrealized gains (losses) on securities available-for-sale, net of tax	46,573	23,387
Foreign currency translation adjustment, net of tax	(14)	4,973
Total stockholders' equity	1,027,224	902,212
Total liabilities and stockholders' equity	$ 1,294,919	$ 1,169,962

Sum of the carrying values as of the balance sheet date of liabilities incurred and payable to vendors for goods and services received, and other costs not separately disclosed in the balance sheet that are statutory in nature, are incurred on contractual obligations, or accumulate over time and for which invoices have not yet been received or will not be rendered.

No definition available.