Significant Accounting Policies - Prospectively Adopted New Accounting Pronouncements (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Item Effected [Line Items]
Increase in commission and other acquisition expenses due to ASU adoption	$ 492,031	$ 438,812	$ 336,697
Deferred commission and other acquisition expenses	270,669	248,436
Accounting Standards Update 2010-26
Item Effected [Line Items]
Increase in commission and other acquisition expenses due to ASU adoption	2,033	2,614
Decrease in Earnings per Share due to ASU adoption, in dollars per share	$ 0.03
Deferred commission and other acquisition expenses		$ 2,614