Investment Securities (Debt securities issued by corporation) (Details) (USD $)	Sep. 30, 2013		Dec. 31, 2012
Investment securities
Fair value investments	$ 1.3
Debt securities issued by corporations
Investment securities
Fair value investments	2,317,200,000	[1]	2,385,100,000	[1]
AAA | Debt securities issued by corporations
Investment securities
Fair value investments	0		0
AA | Debt securities issued by corporations
Investment securities
Fair value investments	216,800,000		193,400,000
A | Debt securities issued by corporations
Investment securities
Fair value investments	1,074,600,000		1,061,000,000
BBB | Debt securities issued by corporations
Investment securities
Fair value investments	1,012,800,000		1,116,900,000
BB | Debt securities issued by corporations
Investment securities
Fair value investments	6,900,000		7,000,000
Other | Debt securities issued by corporations
Investment securities
Fair value investments	$ 6,100,000		$ 6,800,000

[1] Credit ratings are assigned based on the following hierarchy: 1) Standard and Poor's Financial Services LLC ("Standard & Poor’s") and 2) Moody’s Investor Service ("Moody's").