Business Acquisition (Details 1) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011
Business Acquisition, Pro Forma Information [Abstract]
Premium revenue	$ 2,327,573	$ 1,765,710	$ 4,412,381	$ 3,538,352
Net income	$ 31,917	$ 44,450	$ 64,048	$ 113,640
Basic earnings per share	$ 0.67	$ 0.92	$ 1.36	$ 2.35
Diluted earnings per share	$ 0.63	$ 0.83	$ 1.25	$ 2.16