Debt and Financing Arrangements (Financing Structure) (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014		Dec. 31, 2013
Components of financing structure:
Outstanding, total	$ 1,316,545	[1]	$ 1,594,017	[1]
Balance, debt	818,015	[2]	798,499	[2]
7.5% Senior notes due 2016
Components of financing structure:
Outstanding, debt	500,000	[1]	500,000	[1]
Balance, debt	499,690	[2]	499,512	[2]
5.5% Senior notes due 2020
Components of financing structure:
Outstanding, debt	300,000	[1]	300,000	[1]
Balance, debt	299,112	[2]	298,987	[2]
Swiss office building mortgage
Components of financing structure:
Outstanding, debt	14,157	[1]
Balance, debt	14,157	[2]
Swiss office building credit facility
Components of financing structure:
Outstanding, credit facility	5,056	[1]
Balance, credit facility	5,056	[1]
$1,000,000 secured letter of credit facility - uncommitted
Components of financing structure:
Outstanding, credit facility	497,332	[1]	707,915	[1]
Credit facility, maximum borrowing capacity	1,000,000		1,000,000
$150,000 secured letter of credit facility - committed
Components of financing structure:
Outstanding, credit facility	0	[1],[3]	86,102	[1]
Credit facility, maximum borrowing capacity	$ 150,000		$ 450,000

[1] Indicates utilization of commitment amount, not drawn borrowings.

[2] Represents the principal amount borrowed, net of unamortized discount.

[3] The commitment was reduced from $450,000 to $150,000 during 2014.