Employee compensation Restricted Stock Option Awards (Details 2) (USD $) In Millions, except Share data, unless otherwise specified	3 Months Ended				6 Months Ended				12 Months Ended
	Jun. 30, 2014		Jun. 30, 2013		Jun. 30, 2014		Jun. 30, 2013		Dec. 31, 2013
Weighted-average contractual term
Tax deficiency related to vested stock awards	$ 0.1		$ 0.1		$ 0.2		$ 0.4
Employee Stock Option
Restricted stock option awards
Outstanding at the beginning of the period (in shares)					1,229,000
Granted (in shares)					336,000
Exercised (in shares)			0	[1]	(91,000)	[1]	0	[1]
Expired or canceled (in shares)					0
Forfeited (in shares)					(13,000)
Outstanding at the end of the period (in shares)	1,461,000				1,461,000				1,229,000
Vested and exercisable at end of period (in shares)	355,000	[2]			355,000	[2]
Vested, exercisable, and expected to vest at end of period (in shares)	1,428,000	[3]			1,428,000	[3]
Weighted-average exercise price (per option)
Outstanding at the beginning of the period (in dollars per share)					$ 19.32
Granted (in dollars per share)					$ 25.60
Exercised (in dollars per share)					$ 20.02	[1]
Expired or cancelled (in dollars per share)					$ 0.00
Forfeited (in dollars per share)					$ 19.61
Outstanding at end of the period (in dollars per share)	$ 20.72				$ 20.72				$ 19.32
Exercisable, weighted average exercise price (in dollars per share)	$ 20.35	[2]			$ 20.35	[2]
Vested, exercisable, and expected to vest at end of period, weighted average exercise price (in dollars per share)	$ 20.71	[3]			$ 20.71	[3]
Weighted-average contractual term
Outstanding at the beginning of the period					8 years 7 months 20 days				8 years 9 months 26 days
Granted					9 years 7 months 28 days
Exercised					8 years 2 months 20 days	[1]
Outstanding at the end of the period					8 years 7 months 20 days				8 years 9 months 26 days
Vested and exercisable at the end of the period					8 years 2 months 1 day	[2]
Vested, exercisable, and expected to vest at end of period					8 years 7 months 20 days	[3]
Tax deficiency related to vested stock awards	0.1				0.2
Aggregate intrinsic value of vested and exercisable options	3.8				3.8
Aggregate intrinsic value of vested, exercisable and expected to vest options	$ 14.7				$ 14.7

[1] The exercise of stock option awards could result in federal and state income tax expense or benefits related to the difference between the fair value of the stock option at the date of grant and the intrinsic value of the stock option when exercised. The Company recognized income tax expense of $0.1 million and $0.2 million during the three and six months ended June 30, 2014, respectively, related to stock options, which were recorded as adjustments to deferred income taxes. No stock options were exercised during the three and six months ended June 30, 2013.

[2] The aggregate intrinsic value of vested and exercisable options at June 30, 2014 was $3.8 million.

[3] The aggregate intrinsic value of vested, exercisable and expected to vest options at June 30, 2014 was $14.7 million.