Earnings per Share and Unit (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Document Fiscal Year Focus									2015
Current Fiscal Year End Date									--12-31
Weighted Average Limited Partnership Units Outstanding, Basic									154,170	157,950	171,886
Numerator [Abstract]
Net income (loss) attributable to common stockholders	$ 17,609	$ 53,731	$ 32,480	$ 25,174	$ 73,515	$ 47,942	$ 25,482	$ 19,389
Income per common share - basic (note 15):
Net income attributable to common stockholders (in dollars per share)	$ 0.18	$ 0.57	$ 0.35	$ 0.27	$ 0.79	$ 0.52	$ 0.28	$ 0.21
Income per common share - diluted (note 15):
Net income (loss) attributable to common stockholders (in dollars per share)	$ 0.18	$ 0.57	$ 0.34	$ 0.27	$ 0.79	$ 0.52	$ 0.28	$ 0.21
Parent Company [Member]
Numerator [Abstract]
Income (Loss) from Discontinued Operations, Net of Tax, Attributable to Noncontrolling Interest									$ 0	$ 0	$ 121
Income (Loss) from Discontinued Operations, Net of Tax, Attributable to Parent									0	0	65,164
Income (Loss) from Continuing Operations Attributable to Noncontrolling Interest									2,487	1,457	1,360
Income (Loss) from Continuing Operations Attributable to Parent									150,056	187,390	84,640
Preferred Stock Dividends and Other Adjustments									21,062	21,515	21,510
Less: preferred stock dividends									21,062	21,062	21,062
Net income attributable to noncontrolling interests									2,487	1,457	1,481
Net income (loss) attributable to common stockholders									128,994	165,875	128,294
Net (loss) income for common stockholders - diluted									$ 128,994	$ 165,938	$ 128,339
Denominator [Abstract]
Weighted average common shares/units outstanding for basic EPS/EPU									94,391,000	92,370,000	91,383,000
Weighted average common shares/units outstanding for diluted EPS/EPU									94,856,000	92,404,000	91,409,000
Income per common share - basic (note 15):
Continuing operations (in dollars per share)									$ 1.37	$ 1.80	$ 0.69
Discontinued operations (in dollars per share)									0.00	0.00	0.71
Net income attributable to common stockholders (in dollars per share)									1.37	1.80	1.40
Income per common share - diluted (note 15):
Continuing operations (in dollars per share)									1.36	1.80	0.69
Discontinued operations (in dollars per share)									0.00	0.00	0.71
Net income (loss) attributable to common stockholders (in dollars per share)									$ 1.36	$ 1.80	$ 1.40
Earnings Per Common Unit - Diluted [Abstract]
Income from continuing operations									$ 116,937	$ 133,770	$ 84,297
Partnership Interest [Member]
Numerator [Abstract]
Income (Loss) from Discontinued Operations, Net of Tax, Attributable to Noncontrolling Interest									0	0	121
Income (Loss) from Discontinued Operations, Net of Tax, Attributable to Parent									0	0	65,164
Income (Loss) from Continuing Operations Attributable to Parent									150,296	187,709	84,916
Preferred Unit Dividends And Other Adjustments									21,062	21,515	21,510
Net income attributable to noncontrolling interests									2,247	1,138	1,205
Net income (loss) attributable to common stockholders									129,234	166,194	128,570
Net (loss) income attributable to common unit holders									129,234	166,647	129,018
Net (loss) income for common stockholders - diluted									$ 129,234	$ 166,257	$ 128,615
Denominator [Abstract]
Weighted average common shares/units outstanding for basic EPS/EPU									94,546,000	92,528,000	91,555,000
Weighted average common shares/units outstanding for diluted EPS/EPU									95,011,000	92,562,000	91,581,000
Earnings Per Common Unit, Basic [Abstract]
Continuing operations (in dollars per share)									$ 1.37	$ 1.80	$ 0.69
Discontinued operations (in dollars per share)									0.00	0.00	0.71
Net income attributable to common unit holders (in dollars per share)									1.37	1.80	1.40
Earnings Per Common Unit - Diluted [Abstract]
Continuing operations (in dollars per share)									1.36	1.80	0.69
Discontinued operations (in dollars per share)									0.00	0.00	0.71
Net Income (Loss), Net of Tax, Per Outstanding Limited Partnership Unit, Diluted									$ 1.36	$ 1.80	$ 1.40
Income from continuing operations									$ 116,937	$ 133,770	$ 84,297
Continuing operations | Parent Company [Member]
Numerator [Abstract]
Net income (loss) attributable to common stockholders									128,994	165,875	63,130
Net (loss) income for common stockholders - diluted									128,994	165,938	63,175
Continuing operations | Partnership Interest [Member]
Numerator [Abstract]
Income (Loss) from Continuing Operations Attributable to Noncontrolling Interest									2,247	1,138	1,084
Net income (loss) attributable to common stockholders									129,234	166,194	63,406
Net (loss) income for common stockholders - diluted									$ 129,234	$ 166,257	$ 63,451