Earnings Per Share (Schedule of Earnings Per Share, Basic and Diluted) (Details) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended				9 Months Ended
	Sep. 30, 2014		Sep. 30, 2013		Sep. 30, 2014		Sep. 30, 2013
Earnings Per Share, Basic and Diluted [Line Items]
Net income	$ 67,815		$ 56,181		$ 94,806		$ 202,325
Weighted average common shares outstanding (shares)	75,082,852	[1]	65,129,442	[1]	74,030,290	[1]	64,239,117	[1]
Effect of dilutive securities: 2015 warrants (shares)	1,230,020		1,388,826		1,730,077		462,942
Denominator for earnings per common share - assuming dilution (shares)	79,467,110		74,559,716		79,477,258		72,459,076
Earnings per common share	$ 0.90		$ 0.86		$ 1.28		$ 3.15
Earnings per common share - assuming dilution	$ 0.85		$ 0.75		$ 1.19		$ 2.79
Convertible Senior Notes [Member]
Earnings Per Share, Basic and Diluted [Line Items]
Effect of dilutive securities: convertible senior notes (shares)	1,982,089		6,601,347		2,485,974		6,601,347
Stock Options and Deferred Compensation Agreements [Member]
Earnings Per Share, Basic and Diluted [Line Items]
Effect of dilutive securities: share-based payment arrangements (shares)	1,115,927		1,440,101		1,185,973		1,155,670
Restricted Stock Units [Member]
Earnings Per Share, Basic and Diluted [Line Items]
Effect of dilutive securities: share-based payment arrangements (shares)	56,222		0		44,944		0

[1] Weighted average common shares outstanding include shares vested under the NMO Deferred Compensation Plan and exclude unallocated shares held by the ESOP.