Earnings Per Share (Schedule of Earnings Per Share, Basic and Diluted) (Details) - USD ($) $ / shares in Units, $ in Thousands		3 Months Ended		6 Months Ended
		Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Earnings Per Share, Basic and Diluted [Line Items]
Net income - numerator for earnings per common share		$ 82,845	$ 36,744	$ 88,748	$ 26,991
Weighted average common shares outstanding (shares)	[1]	77,237,200	74,461,264	77,139,992	73,495,286
Effect of dilutive securities: 2015 warrants (shares)		779,926	1,595,519	793,132	1,981,815
Denominator for earnings per common share - assuming dilution (shares)		79,227,317	79,517,804	79,173,143	79,582,763
Earnings per common share		$ 1.07	$ 0.49	$ 1.15	$ 0.37
Earnings per common share - assuming dilution		$ 1.05	$ 0.46	$ 1.12	$ 0.34
Convertible Senior Notes [Member]
Earnings Per Share, Basic and Diluted [Line Items]
Effect of dilutive securities: convertible senior notes (shares)		0	2,259,425	0	2,843,987
Stock Options and Deferred Compensation Agreements [Member]
Earnings Per Share, Basic and Diluted [Line Items]
Effect of dilutive securities: share-based payment arrangements (shares)		1,087,893	1,154,726	1,123,614	1,220,996
Restricted Stock and Restricted Stock Units [Member]
Earnings Per Share, Basic and Diluted [Line Items]
Effect of dilutive securities: share-based payment arrangements (shares)		122,298	46,870	116,405	40,679

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