Investments (Details) (USD $)	3 Months Ended		6 Months Ended			6 Months Ended		78 Months Ended			6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013	Dec. 31, 2013	Jun. 30, 2014 Auction rate securities	Jun. 30, 2013 Auction rate securities	Jun. 30, 2014 Auction rate securities	Dec. 31, 2013 Auction rate securities	Mar. 31, 2008 Auction rate securities	Jun. 30, 2014 Auction rate securities Minimum	Jun. 30, 2014 Auction rate securities Maximum
Schedule of Available-for-sale Securities [Line Items]
Maturity period of securities											17 years	32 years
Auction rate securities at par value	$ 751,899,000		$ 751,899,000		$ 713,950,000	$ 11,025,000		$ 11,025,000	$ 10,898,000	$ 82,100,000
Auction rate securities at par value, settled								70,700,000
Pretax unrealized gains	$ 391,000	$ (4,045,000)	$ 1,817,000	$ (3,626,000)		$ 100,000	$ 400,000